That unmistakable rotten egg smell coming from your bathroom drain is almost always caused by hydrogen sulphide gas — a by-product of bacteria breaking down organic matter like hair, soap scum, and skin cells trapped in your drain or pipes. The good news is that in most cases this is a DIY-fixable problem, though occasionally it signals something that needs a licensed professional.
What Causes the Rotten Egg Smell?
There are a few common culprits:
- Biofilm build-up – A slimy layer of bacteria, hair, and soap residue coats the inside of your drainpipe and produces hydrogen sulphide gas as it decomposes.
- Dry or blocked P-trap – The curved section of pipe beneath your basin or shower (the P-trap) holds a small amount of water that acts as a seal, blocking sewer gases from rising up. If a drain hasn't been used for a while, this water evaporates and gas escapes.
- Blocked or poorly vented drain – A partial blockage can trap decomposing material, and inadequate venting can allow sewer gases to be drawn back through the trap.
- Issues with the sewer or septic system – Less commonly, the smell may originate further down the line.
Step 1 – Clean the Drain Thoroughly
Start with the most likely cause: biofilm and gunk in the drain.
- Remove the drain cover (usually just lifts or unscrews) and pull out any visible hair and debris — gloves are a good idea here.
- Flush with boiling water – Carefully pour a full kettle of boiling water down the drain to loosen grease and soap scum. Note: avoid this on PVC pipes that may be older or fragile; use very hot tap water instead.
- Baking soda and white vinegar – Pour half a cup of baking soda down the drain, followed by half a cup of white vinegar. Let it fizz for 15–20 minutes, then flush with hot water. This is a safe, effective and eco-friendly method.
- Use a drain brush or flexible cleaning tool – A long bottle brush or drain snake from Bunnings or Mitre 10 can physically scrub the inside of the drainpipe and remove the biofilm causing the smell.
- Enzyme-based drain cleaner – These products use beneficial bacteria to break down organic matter without damaging pipes or harming the environment. Look for them at hardware stores. Avoid harsh chemical drain cleaners as a first resort — they can damage older pipes and are harmful to waterways.
Step 2 – Check and Refill the P-Trap
If a bathroom hasn't been used for a few weeks (a guest bathroom, for example), the P-trap may have dried out.
- Simply run the tap for 30–60 seconds to refill the water seal.
- If the smell returns quickly after refilling, your drain may have a venting problem that is drawing the water out of the trap — this needs a licensed plumber to assess.
Step 3 – Clean the Overflow and Drain Cover
Basins often have an overflow hole near the top — this can harbour a surprising amount of bacteria and is frequently overlooked. Use an old toothbrush and a diluted bleach solution or white vinegar to scrub it out. Remove and scrub the drain cover itself as well.
Step 4 – Establish a Regular Maintenance Routine
Prevention is much easier than cure. Try to:
- Use a drain strainer/hair catcher to stop hair and debris entering the drain in the first place.
- Do a baking soda and vinegar flush once a month.
- Run water through infrequently used drains every couple of weeks to keep P-traps full.
- Pour a small amount of cooking oil down rarely used drains — it sits on top of the water in the P-trap and may help slow evaporation. Note that regularly pouring oil down drains can contribute to blockages over time, so use this sparingly.
Could It Be a Bigger Problem?
If you've cleaned the drain thoroughly and the smell persists, consider these possibilities:
| Symptom | Likely Cause | Action |
|---|---|---|
| Smell returns within days of cleaning | Venting issue or partial blockage deeper in the pipe | Call a plumber or drainlayer |
| Multiple drains smell at once | Sewer or septic system issue | Call a licensed drainlayer urgently |
| Gurgling sounds with the smell | Blocked or poorly vented drain | Call a plumber |
| Smell only when it rains | Possible septic or stormwater issue | Call a drainlayer |
If you're on a septic tank system, a rotten egg smell can sometimes indicate the tank needs pumping or servicing — contact a septic specialist.
Safety Note
Hydrogen sulphide gas is toxic in high concentrations. While the levels typically found in a smelly bathroom drain are not dangerous, if the smell is very strong, persistent throughout the house, or accompanied by dizziness or nausea, ventilate the area immediately and contact a licensed professional. WorkSafe NZ has guidance on hydrogen sulphide hazards.
When to Call a Professional
Drainlaying and certain plumbing work in New Zealand is restricted work under the Plumbers, Gasfitters, and Drainlayers Act 2006. You must use a licensed tradesperson for:
- Any work on the drains beyond the fixture trap (i.e., the pipework inside the walls or underground)
- Diagnosing and fixing venting problems
- Clearing deep or stubborn blockages with specialist equipment
- Any work connecting to the public sewer
Check that your tradesperson is registered with the Plumbers Gasfitters & Drainlayers Board. You can find qualified local drain cleaners and plumbers on tradedirectory.co.nz.
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