A smelly drain almost always comes down to one of three things: a dry trap letting sewer gas into the room, a build-up of grease and grime inside the pipe, or a genuine blockage further down the line.
What it smells like usually tells you which one you've got, well before a plumber needs to confirm it. This guide is organised by smell type rather than by which room the drain is in, since a musty smell, a rotten-egg smell, and a sewage smell each point to genuinely different problems.
Quick Diagnosis: What Does the Smell Tell You?
Work out the likely cause and urgency before doing anything else:
| Smell | Likely cause | Urgency |
|---|---|---|
| Musty or mouldy | Biofilm, grease, hair and soap scum breaking down in the pipe | Low |
| Rotten egg / sulphur | Dry trap, the water seal that blocks sewer gas has evaporated | Low |
| Sewage, from one drain | Blockage or venting issue in that branch line | Medium |
| Sewage, from multiple drains at once | Vent stack or main sewer line problem | Call a plumber |
Musty or Mouldy Smell
A musty or mouldy smell is usually biofilm, a mix of grease, soap scum, hair and organic matter that builds up on the inside of the pipe and starts to break down. It's the most common drain smell overall, most likely in drains used daily like kitchen sinks and shower drains. Rarely urgent, but it won't clear on its own.
- A hot water and detergent flush is usually enough for a mild case.
- Smell returns within a week or two? The buildup is likely further down than a surface flush can reach.
- At that point, a professional clean beats repeated DIY attempts.
Rotten Egg or Sulphur Smell
A rotten egg or sulphur smell almost always means a dry trap, the U-shaped section of pipe that holds a permanent water seal to block sewer gas from entering the room.
The plumbing code (AS/NZS 3500.2) requires every trap to hold a minimum 25mm water seal at all times, specifically to stop gas and odour passing through. Traps dry out through simple evaporation, fastest in drains that aren't used often, like laundry floor wastes or a guest bathroom, especially over summer.
The fix is usually simple: pour a litre of water down the drain to refill the seal. If the smell returns within a few days rather than weeks, the trap may have a slow leak rather than just evaporation, and it's worth having checked.
Sewage Smell From One Drain
A sewage-like smell from a single drain usually points to a blockage or venting issue in that specific branch line. As waste builds up in a pipe, trapped gas has nowhere to go except back up through the fixture. This is different from a rotten-egg smell, typically stronger, more persistent, and often paired with slow draining or gurgling when the fixture is used.
Slow draining or gurgling alongside the smell is a strong sign of a partial blockage rather than a dry trap. Our blocked drains page covers common causes and what can be safely cleared at home versus what needs professional equipment.
Sewage Smell From Multiple Drains at Once
A sewage smell from several drains at the same time points to something bigger, most often the property's venting or the main sewer line, and it's not a DIY situation.
Plumbing systems rely on vent stacks to let sewer gas escape safely above the roofline. If those vents are blocked, damaged, or poorly installed, gas can be forced back through multiple fixtures at once, particularly noticeable after heavy rain or when several appliances run together.
This can also indicate a partial blockage or damage further along the main drain, affecting everything that feeds into it. A CCTV drain inspection is the fastest way to find the exact cause without guesswork or unnecessary digging.
See our drainage inspection and sewer camera inspection pages for how that works.
Simple Checks You Can Do at Home
Before assuming the worst, a few quick checks can narrow down the cause on their own.
When a Drain Smell Means You Need a Plumber
Most drain smells are a quick DIY fix, but a few signs mean it's time to bring in a professional.
- The smell returns within days of a DIY fix.
- More than one drain smells at the same time.
- The smell is paired with gurgling, slow draining, or water backing up.
- You can smell sewage outside near the boundary trap or inspection point.
Preventing Smelly Drains Long-Term
A short, regular routine keeps most of these causes from coming back. Run water through rarely-used drains every couple of weeks, avoid pouring fats or oils down the kitchen sink, and use drain strainers to catch hair and food scraps before they build up. For anything beyond that, our drainage services page covers ongoing maintenance options.
Smell Keeps Coming Back?
Get in touch with DMACK Plumbing & Gas and we'll find the actual cause, not just mask the symptom.
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