Blocked Drains

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 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: How to confirm it’s a dry trap 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: 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 If you see or feel: 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 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