Why Your Drains Smell & How to Fix It

Most of us spend a lot of time keeping our sinks, showers, and benches clean, but what about the drains? Even when everything else looks tidy, hair, soap scum, food scraps, and grease can quietly build up inside pipes, causing smells that are hard to ignore.

Fortunately, most drain odours give clues about where the problem is and what it is. By paying attention to the smell and which drain it's coming from, you can often tackle the issue yourself before it turns into a bigger headache.

In this guide, we'll walk you through why drains smell, show you practical steps to fix common problems at home, and explain when it's time to call a licensed plumber to keep your system running smoothly.

TL;DR

• Common causes include dry P-traps, biofilm build-up, hidden overflow channels, food residue in kitchen pipes, partial clogs, blocked vents, or issues with the main sewer line.
  
  
• To fix it, identify the odour type first, as raw sewage, rotten eggs, musty/mouldy, or rotting food odours can indicate different plumbing issues.
  
  
• Perform DIY fixes for minor problems. Refill traps, clean drain surfaces and overflow channels, break down biofilm, remove hair/debris, and check kitchen hoses and garbage disposal.
  
  
• Avoid pouring harsh chemicals, mixing cleaners, flushing wipes, using grease, or boiling water in PVC pipes, as these can worsen the problem.
  
  
• Do regular maintenance, such as weekly trap top-ups, hot-water flushes, hair removal, monthly enzymatic cleaners, and kitchen/overflow checks, to keep drains fresh.
  
  
• Call a licensed plumber if there are persistent odours across multiple drains, gurgling toilets, sewage backups, or recurring smells after cleaning.

Identify the Smell: A Quick Odour-to-Cause Cheat Sheet

Before you start cleaning drains or checking pipes, pause for a moment and pay attention to the smell.

Different plumbing problems produce different odours. The type of smell can often tell you whether you're dealing with sewer gas, bacterial build-up, mould, or food waste, which helps narrow down the likely cause.

Use the quick guide below to match the smell with what it usually points to. 

If It Smells Like Raw Sewage

A strong sewer smell is the easiest one to recognise. It has a heavy, unpleasant odour similar to wastewater or public sewer drains.

When you notice this smell indoors, it means sewer gases are entering the home somewhere in the plumbing system. These gases normally stay trapped inside the sewer lines, held back by the water in the P-trap (the U-shaped section under sinks, showers, and floor drains). If the P-trap dries out, is damaged, or the seal is otherwise broken, the gases can escape into your living space.

If It Smells Like Rotten Eggs or Sulphur

A rotten-egg odour comes from hydrogen sulphide gas, which forms when bacteria feed on organic material – like hair, soap residue, or other debris – inside drains.

This smell often becomes stronger when water runs because moving water disturbs the bacteria and the trapped gases in the P-trap or deeper in the drain. Over time, this bacterial buildup can lead to slow-draining pipes, persistent odours, and even corrosion in metal pipes if left untreated.

If It Smells Musty or Mouldy

A musty, damp odour is softer and earthier, more like wet towels or old cupboards, rather than the sharp smell of sewer gas.

This happens when moisture lingers for long periods. In bathrooms, warm, humid air combined with splashes from sinks and showers keeps surfaces damp, which encourages mould and bacteria to grow and release musty smells. These smells can also come from damp buildup in overflow holes or around drain openings, where water doesn't flow freely and bacteria or mould can thrive.

If It Smells Like Rotting Food or Garbage

A garbage-like odour is sharp, sour, and slightly acidic, similar to leftover food left sitting in a bin. It often comes from decomposing vegetables, meat scraps, or greasy residues.

The smell is usually strongest near the kitchen sink, garbage disposal, or dishwasher because food scraps and grease get stuck in these areas and begin to rot.

The Most Common Causes of Smelly Drains (And Quick Ways to Confirm Them)

Drains can start smelling for a few common reasons, mostly related to buildup or trapped water. Knowing what's causing it makes it easier to fix.

Cause 1: Evaporated Water in the P-trap 

Every drain in your home connects to a P-trap, which is the U-shaped section of pipe under sinks, showers, and floor drains. That curved section always holds a small amount of water, usually about 50–100 mm (2–4 inches), which acts as a seal to block sewer gases from travelling up the pipe and into your home.

If a drain isn't used for a while, that water slowly evaporates. In warm climates like Townsville, traps can dry out faster because heat speeds up evaporation. Once the trap dries out, there's nothing stopping sewer gases from rising through the pipe.

This often happens in places that aren't used daily, such as:

• guest bathrooms
  
  
• laundry floor drains
  
  
• garage floor wastes
  
  
• outdoor sinks

How to confirm it's a dry trap

1. Identify the drain where the smell is strongest.
   
   
2. Pour about 1–2 litres of water directly down that drain.
   
   
3. Wait 5–10 minutes for the water to settle in the trap.
   
   
4. Smell the area again. 

• If the smell disappears completely, the issue was simply a dry trap, and the water seal has now been restored. 
• If the smell returns quickly, the trap may be losing water for other reasons, which we will discuss below.

Cause 2: Biofilm Buildup Inside The Drain

Another common cause of smelly drains is biofilm, the thin, slimy layer that slowly forms inside pipes when everyday materials stick to the pipe wall, including:

• soap residue
  
  
• toothpaste
  
  
• shampoo and conditioner
  
  
• hair
  
  
• skin oils

Bacteria feed on this organic material and form a sticky colony. As they break it down, they release gases that create musty or sulphur-like smells.

How to confirm biofilm buildup

1. Remove the drain cover and look inside the drain opening.
   
   
2. Use a cotton swab, tissue, or an old toothbrush to gently swipe the inside edge of the pipe or drain grate.
   
   
3. Check what comes out.

If you see or feel:

• a slimy or jelly-like coating, or
  
  
• dark or pink residue that smells stronger than the air in the room

Then, biofilm is likely coating the inside of the drain.

Cause 3: The Hidden Sink Overflow Channel

Many bathroom basins have a small hole near the top of the bowl called the overflow. Its job is to stop the sink from flooding if the tap is left running. If the basin fills too high, water flows through this hole and back into the drain.

This overflow channel rarely gets properly flushed because, during normal use, water goes straight down the main drain rather than through the overflow opening. As a result, small splashes of toothpaste, soap, dust, hair, and dirty water can enter the hole and remain inside the narrow cavity instead of being washed away.

Because the channel stays dark and slightly damp, bacteria and mould begin feeding on the residue. As they break it down, they release the musty or sour smells you often notice when leaning over the basin.

How to confirm the overflow is the source

1. Plug the sink drain.
   
   
2. Fill the basin until water starts entering the overflow hole.
   
   
3. As the water flows through the channel, notice whether the smell becomes stronger.

If the odour becomes stronger, it means the moving water is disturbing the buildup inside and releasing the trapped smell into the air.

Cause 4: Organic Buildup in the Kitchen (FOG)

Kitchen drains deal with a different type of waste than bathroom drains.

Even if you're careful not to pour grease down the sink, small amounts of fats, oils, and grease (FOG) still wash off plates, pans, and utensils during washing.

This greasy residue travels down the pipe, cools, and sticks to the walls. Over time, it traps small food scraps, rice, pasta, vegetable bits, or sauce residue. Bacteria then feed on this buildup, producing the sour, garbage-like odours that linger near your kitchen drains.

How to confirm organic buildup in the kitchen drain

1. Stand near the sink and run the tap for about 30–60 seconds. If the smell becomes stronger while water is flowing, it means the odour is coming from inside the kitchen drain pipe.
   
   
2. Look into the drain opening and check for grease, food bits, or slimy residue clinging to the edges. If you see any of these, the odour is likely coming from organic material trapped inside the kitchen drain.

Cause 5: Partial Clogs and Stagnant Pockets

Sometimes a drain can smell even though water still flows normally. This happens when a partial clog forms inside the pipe. Instead of completely blocking the pipe, materials like hair, lint, soap residue, or grease build up along the pipe walls, narrowing the opening.

Water can pass through, but debris collects in slow-moving spots, creating stagnant pockets. In these areas, organic material breaks down and produces unpleasant odours. Because the pipe isn't fully blocked, many homeowners don't notice the clog until the smell appears.

How to confirm a partial blockage

1. Run the tap and watch how the drain empties. If the water drains but slightly slower than usual, it can indicate a build-up inside the pipes.
   
   
2. Listen closely for gurgling sounds when the water finishes draining.
   
   
3. Pay attention to the smell immediately after the water finishes draining.

If you hear gurgling and the odour becomes stronger right after the water drains away, it often means water is moving past a narrowed section of pipe where debris has collected. As the water passes through that restricted area, it disturbs the trapped debris and releases the smell from the buildup.

Cause 6: Venting Issues (A Blocked Vent Stack)

Your plumbing relies on air moving through the system to help wastewater flow smoothly. This airflow comes from vent pipes, often called the vent stacks, which run from your drains up through the roof. If a vent becomes blocked by leaves, debris, or nests, the system can't draw in air properly. 

When large volumes of water drain (like when flushing a toilet or emptying a bath), the pipes may try to draw air from the nearest available source, such as P-traps in sinks, showers, or floor drains. When this happens, the water in the trap may bubble or partially drain, weakening the seal that blocks sewer gas. Once that seal is disturbed, sewer gases can travel up the pipe and create strong odours inside the house.

How to confirm a venting problem

1. Run water in one fixture, like a bathroom sink.
2. Watch what happens in other fixtures.

• Gurgling sounds from sinks, showers, or floor drains.
  
• Toilet bowl water moves or bobs.
  
• Sewer smells return quickly, even after drains have been cleaned.

If several fixtures show these signs at the same time, it means a vent blockage is affecting airflow throughout the drainage system. 

Cause 7: Trouble in the Main Sewer Line

If the smell is appearing from multiple drains across the house at the same time, the problem may be in the main sewer line. 

When this pipe becomes partially blocked, wastewater cannot move away from the house as quickly as it should. It slows down inside the pipe and begins to sit in sections of the line. As that wastewater sits, it produces strong sewer gases that can travel back through several household drains.

How to confirm a main sewer line problem

1. Walk around the house and sniff sinks, showers, and floor drains. If more than one drain is producing odour at the same time, the problem is likely in the main line.
   
   
2. Run water in a single fixture. Turn on a tap, flush a toilet, or run the washing machine.
   
   
3. Observe other fixtures while the water drains. Look for signs like toilets bubbling, water rising in floor drains or showers, or unusual gurgling sounds from nearby sinks. These show that wastewater is struggling to flow through a blocked main sewer line.

Another important place to check is the overflow relief gully (ORG) outside the house. This is the grated drain located near the laundry or kitchen wall. Its job is to take wastewater and spill it outside to prevent sewage from backing up inside your home if the main sewer line is blocked. 

The ORG normally stays dry because wastewater flows freely through the main sewer line. If you see water or sewage spilling out from the ORG, it indicates a blockage further down the main pipe.

DIY Fixes: Step-by-Step Solutions

Once you have a clearer idea of what might be causing the smell, you can normally fix it with a few easy steps. Start with the simplest things first, then move deeper into the system only if the smell persists.

Step 1: Refill Rarely Used Drains

Start by running the tap or shower connected to the drain that smells. Let the water run for 20–30 seconds to fill the P-trap, which blocks sewer gases. This top-up restores the water seal and can often stop the odour right away.

For floor drains, pour 5–10 litres of water slowly into the grate (the cover over the drain) to refill the trap underneath. Wait a few seconds and take a sniff. If the odour disappears, the problem was simply a dry trap, and the water seal is now restored.

Step 2: Clean the Surface Where Smells Start

Even if the P-trap is full, odours can still come from the surfaces right at the drain opening because hair, soap scum, toothpaste, and grease often stick in the first few centimetres of the pipe, where they can start to decompose and release a concentrated smell.

To tackle this:

• Remove the sink stopper, shower grate, or garbage-disposal splash guard to properly reach the drain.
  
  
• Use an old toothbrush with dish soap to scrub the underside and edges, focusing on any dark, sticky, or slimy spots.
  
  
• Rinse the area with hot water (around 60 °C) to wash away the loosened residue.

Step 3: Flush Hidden Overflow Channels

If, even after cleaning the surface, smells linger, it may mean there is a build-up in the hidden overflow channels near the tops of bathroom sinks.

 

To clean it:

1. Insert a small funnel into the overflow hole.
   
   
2. Pour about 2 cups of white vinegar into the opening and let it sit for 10–15 minutes. The vinegar helps loosen residue and slows bacterial growth.
   
   
3. Follow with a kettle of hot (not boiling) water to flush the loosened build-up through the channel.

Step 4: Break Down Bacterial Slime Inside the Drain

If the odour persists after cleaning the hidden overflow channels, it is often coming from a slimy layer deeper in the pipe – the biofilm.

To tackle it safely:

1. Pour ½ cup of bicarbonate of soda into the drain.
   
   
2. Follow with ½ cup of white vinegar.
   
   
3. Plug the drain so the fizzing action moves downward into the pipe. This fizzing helps loosen the slime and neutralise odours without harsh chemicals.
   
   
4. Leave it for 15 minutes, then flush with a full kettle of hot water.

Step 5: Remove Hair and Debris Physically

Once you've broken down the biofilm, if the smell is still there, it usually means solid debris – like hair, lint, or soap bits – is stuck in the pipe and holding on to the odour.

What to do:

1. Use a zip tool or flexible drain snake.
   
   
2. Feed it slowly into the drain and twist or move it up and down to catch hair and debris.
   
   
3. Pull out the tool and dispose of whatever comes up; you may find clumps of hair and dark slime.
   
   
4. Run hot water down the drain to rinse, then check the smell again.

If the odour was gone, the problem was trapped debris. If it persists, the issue may be further along the pipe system.

Step 6: Check Kitchen Drains and Dishwasher Connections

By now, you've cleared surface debris and biofilm from bathroom drains, so it's time to turn your attention to the kitchen, where smells often come from trapped food, grease, or issues with the dishwasher drainage.

Do this:

1. Check the dishwasher hose: Make sure it loops upward under the benchtop before connecting to the drain. This high loop stops dirty sink water from flowing back into the dishwasher hose and sitting there, which can cause smells.
   
   
2. Clean the garbage disposal splash guard: Lift the rubber flap inside the sink opening and remove any trapped food, grease, or slime underneath.
   
   
3. Clean the dishwasher air gap (if you have one): This is the small chrome cylinder near the tap. Remove the cap and clear any debris inside. It prevents dirty water from flowing back into the dishwasher.
   
   
4. Test the drain: Run the tap briefly and smell near the drain again to see if the odour is gone.

If odours persist after the bathroom and kitchen checks, the problem may not be limited to the drains themselves – it could involve blocked venting or the main sewer line. This is not a DIY fix, so contact a licensed plumber to inspect and clear the blockage safely.

At 1touch Plumbing, we specialise in locating and clearing blocked vent stacks and main sewer lines, using the right tools and techniques to stop odours at their source and prevent further plumbing issues.

What Not To Do: Common Mistakes That Make Smells Worse

When a drain starts to stink, it's natural to want a quick fix, but some quick fixes can actually make the problem worse, damage your pipes, or create a hazardous environment in your home. So please avoid them. 

Do Not Over-Rely on Chemical Drain Openers

Most chemical drain cleaners work by creating a strong chemical reaction that generates intense heat to break down grease or hair. That heat can soften, warp, or even damage the pipe and its glued joints, especially in plastic plumbing. With repeated use, these chemicals can also make pipes brittle, increasing the risk of hidden leaks behind walls and leading to costly repairs.

Another issue is that these liquids are quite heavy. Instead of travelling through the pipe and clearing the buildup, they often settle at the bottom of the P-trap, pressing against the pipe walls without actually reaching the biofilm or hair, causing the smell to worsen.

Never Mix Different Cleaning Products

Mixing common household cleaners can create toxic gases or violent chemical reactions that can harm your health or cause chemicals to splash out of the drain.

For example:

• Bleach + vinegar → produces chlorine gas, which can cause coughing, breathing problems, and severe eye irritation.
  
  
• Bleach + ammonia → creates chloramine vapours, which are highly irritating and can be dangerous in enclosed spaces.
  
  
• Acidic cleaner + caustic cleaner → can react violently and force burning chemicals back out of the drain.

Always use one cleaner at a time and rinse the drain thoroughly with water before using anything else.

Don't Use Flushable Wipes or Grease to Mask Odours

Pouring scented oils or cleaning liquids down the sink to mask odours can make the problem much worse.

Even small amounts that wash down the sink can solidify in the cooler pipes under your house. Over time, this sticky layer traps food debris and other waste, creating a larger blockage that produces stronger odours.

The same issue occurs with wipes. Even products labelled 'flushable' don't break down the way toilet paper does. Instead, they tend to snag inside pipes, especially at joins or bends, where they collect other waste. Over time, this can create a buildup that leads to blockages and, eventually, wastewater backing up into the home.

Skip the Boiling Water Myth For PVC

While many online guides suggest pouring a full kettle of boiling water down the drain, it's safer to let the water cool slightly first. In most Townsville homes, the drains are PVC with solvent-welded joints, which aren't designed to handle 100°C water.

Repeatedly shocking your pipes with boiling water can cause thermal expansion and contraction, which eventually cracks the pipes or joints. Use hot water (around 50–60°C); it's warm enough to melt soap scum but safe enough to keep your plumbing intact.

How to Keep Your Drains Smelling Fresh: Easy Maintenance Steps

By following a few simple habits regularly, you can stop smells before they start and keep your drains fresh.

• Weekly P-Trap Top-Up: For drains you rarely use – guest bathrooms, laundry floor drains, and outdoor sinks – run the tap for 30 seconds or pour 5 litres of water into floor drains once a week. This keeps the P-trap full, maintaining the seal that blocks sewer gas.
  
  
• Kitchen Hot Water Flush: After washing dishes, pour 1 litre of hot tap water (50–60 °C) down the kitchen sink. This moves leftover fats, oils, and small food scraps through the pipes before they cool and stick, reducing grease build-up and odours.
  
  
• Remove Hair and Debris Weekly: Hair is the main anchor for biofilm in showers and vanity drains. Once a week, remove hair from pop-up wastes, grates, and shower drains using a zip tool, toothbrush, or your fingers. This prevents bacterial slime from forming.
  
  
• Monthly Enzymatic Clean: Use an Australian-made enzymatic drain cleaneronce a month. These are eco-friendly cleaners that use live bacteria to digest organic residue inside pipes overnight without damaging the pipes.
  
  
• Scrape Food Before Washing: Don't rely on sink strainers alone. Scrape leftover food, sauces, or dairy into the bin or compost before washing. Even tiny amounts left in the pipe can rot quickly in Townville's heat and cause kitchen odours.
  
  
• Overflow Hole Care: Bathroom sink overflow channels often hold damp residue. Once a month, pour a small amount of vinegar into the overflow and rinse with hot water. You can also use a small bottle brush to remove trapped grime.
  
  
• Garbage Disposal & Dishwasher Check: If you have a disposal or dishwasher, inspect the backsplash guard and hose connections monthly. Remove trapped food debris, and make sure dishwasher drain hoses have a high loop to prevent wastewater from backing up into the appliance.

When to Call a Plumber (Don't DIY These)

If you've tried the DIY steps and the smell persists, it means there's a deeper plumbing or structural issue that you can't fix yourself. 

Call a licensed plumber if you notice any of the following:

• Odours from multiple rooms or all drains at once – If several drains across the house smell at the same time, the problem is likely in your main sewer line rather than a single sink or shower.
  
  
• Toilet bubbling or gurgling sounds – If your toilet bubbles when someone runs the shower, or you hear deep gurgling in the laundry when the kitchen sink drains, the pipes are struggling for air. This often points to a blocked or damaged vent pipe.
  
  
• Greywater or sewage backing up – If water from showers, floor drains, or laundry starts appearing when other taps are running, the main sewer line may be blocked, possibly due to tree roots growing into older pipes or a collapsed section.
  
  
• Smell returns after cleaning drains – If you've cleaned the traps and scrubbed biofilm, but the odour comes back within 24 hours, there may be a cracked or shifted pipe under the concrete slab. In Townsville's reactive clay soils, the ground can move slightly over time, causing pipes to crack or separate, which lets sewer gases escape into the floor or foundations.
  
  
• Persistent mouldy smell with damp spots – If drains look clean, but you still smell mould, and you notice damp patches on walls, soggy soil, or mushy grass near external pipes, there may be a hidden leak somewhere in your plumbing system.

Once the plumber arrives, give them clear information about: 

• Which fixtures smell (e.g., "Just the ensuite shower and the laundry floor waste")
  
  
• When is the smell strongest (e.g., "First thing in the morning" or "Only after the dishwasher runs")
  
  
• Any unusual noises. Tell them about gurgling, whistling, or slow-draining water.
  
  
• Age and type of plumbing: Mention how old your home is and the type of pipes it likely has. Older homes often have earthenware pipes that are vulnerable to root intrusion or cracking, while newer builds usually have PVC pipes, which are less prone to damage but can have venting or installation issues.

Providing these details helps the plumber quickly narrow down the source of the problem, decide what tools or equipment they may need, and fix the issue efficiently without unnecessary digging or guesswork.

Final Thoughts 

Keeping up with regular maintenance is the easiest way to stop unpleasant odours before they even start. That said, even if you're careful, issues can still pop up. When that happens, it's tempting to grab a quick fix or pour harsh chemicals down the drain to mask the smell, but that usually does more harm than good. The real solution is to tackle the root cause, not just cover up the symptom.

Some problems are simple enough to handle yourself, like refilling a dry P-trap. Others, like blocked vents or issues in the main sewer line, are best left to the experts. Knowing the signs and when to call in a professional can save you a lot of stress, time, and money.

That's where we come in. At 1touch Plumbing, our licensed team can quickly identify the cause of your plumbing odours and fix them properly, whether it's a minor blockage or a more complex vent or sewer issue. 

Contact us today to ensure your home always smells fresh!

FAQs

I live in a brand-new house; why do my drains already smell like sewage?

In new homes, a common cause of sewer odours is dry P-traps. Many drains, especially in guest bathrooms, basements, or laundries, haven't been used yet. When the water in a P-trap (the U-shaped section under the drain) evaporates, it breaks the seal that normally blocks sewer gases, letting the odour escape.

To fix it, run water in every sink, shower, and tub for 20–30 seconds to refill the traps. For floor drains, slowly pour a bucket of water into them to restore the seal and stop the smell.

Is it true that coffee grounds can deodorise my smelly drain?

No. Coffee grounds don't dissolve in water. Instead, they sink to the bottom of the P-trap and mix with grease, forming a thick, gritty sludge that can block the pipe and make odours worse. Rather than putting them down the drain, dispose of coffee grounds in the bin or add them to your garden as compost, where they can safely break down and enrich the soil.

Why does my drain smell more during the Townsville wet season?

During heavy tropical rain, the sewer system can become overwhelmed as stormwater enters the pipes. This extra water increases pressure inside the sewer lines. If the water in your drains or P-traps is low, or if vent pipes are partially blocked, this pressure can push sewer gases back into your home, making drains smell worse than usual.

To prevent this, make sure all sinks, showers, tubs, and floor drains have water in their P-traps to block gases, and check that roof vents aren't blocked so air can flow properly.