A drain that smells after heavy rain is a common complaint from NZ homeowners, and the good news is it's usually fixable. The odour is almost always caused by sewer gases — mainly hydrogen sulphide — being pushed or drawn up through your drainage system when stormwater puts pressure on the network. Understanding the cause is the first step to getting rid of that rotten-egg or musty smell for good.
Why Heavy Rain Triggers Drain Smells
When it rains heavily, several things can happen at once that disturb your drainage system:
- Surcharging in the public sewer or stormwater network — nearby pipes fill up fast, creating back-pressure that forces gases up through your household drains.
- Dry P-traps — the U-shaped bend under sinks, basins, and floor wastes holds a small amount of water that acts as a seal against sewer gases. If a drain hasn't been used in a while, that water evaporates, and heavy rain can draw gases straight through.
- Cross-connections between stormwater and wastewater — in older NZ homes, stormwater and sewage pipes were sometimes connected. Heavy rain flooding the stormwater side can push sewage gases (or even raw sewage) into the house.
- Blocked or poorly ventilated drain stacks — your drainage system relies on vent pipes to let air in and gases out safely above the roofline. A blocked vent (leaves, bird nests, debris) creates negative pressure when water rushes through during rain, siphoning water out of traps and letting gases in.
- Cracked or root-damaged pipes — tree roots or ground movement can crack pipes, and heavy rain saturating the soil can push odours through those cracks and up through floor drains or gully traps.
How to Diagnose the Problem
Before reaching for the drain cleaner, spend a few minutes tracking down the source:
- Sniff around — is the smell coming from one drain or multiple? A single smelly drain usually points to a dry trap or local blockage. Multiple drains suggest a venting issue or cross-connection.
- Check your gully trap — the outdoor gully trap (the open receptor where your laundry and kitchen wastewater exits the house) is a common culprit. It should have a water seal. If it's cracked, blocked with debris, or the water seal is broken, it will smell badly after rain.
- Look at floor wastes — floor wastes in bathrooms and garages are often unused and their traps dry out. Pour a bucket of water down them and see if the smell improves.
- Inspect the roof vent — if you can safely access your roof, check that the vent pipe is clear of leaves and debris.
DIY Fixes You Can Try
Refill Dry Traps
The simplest fix: pour about two litres of water down any drain that isn't used regularly — floor wastes, spare bathroom basins, laundry tubs. For traps that dry out repeatedly, you can add a small amount of cooking oil on top of the water to slow evaporation, or fit a trap primer (a device that automatically tops up the trap).
Clean the Gully Trap
Remove the gully trap cover and clear out any leaves, silt, or debris. Flush it through with a hose. If the trap itself is cracked or the seal is broken, it will need replacing — this is straightforward plumbing work.
Clear Debris from Roof Vents
If you can safely access the roof (use appropriate fall protection — see WorkSafe NZ for guidance), remove any blockages from the vent pipe opening. Never work on a roof alone.
Use an Enzyme-Based Drain Cleaner
For smells caused by organic build-up (grease, soap scum, hair) in the pipe itself, an enzyme or bacterial drain cleaner is a gentler and more effective long-term option than harsh chemical cleaners. These are available from Bunnings or Mitre 10. Avoid repeatedly using caustic soda or strong acid cleaners, as these can damage older pipes and are hazardous to handle.
Check for Cross-Connections
If your home was built before the 1990s, there's a chance your stormwater and wastewater systems may be connected — something that doesn't comply with the current NZ Building Code (specifically Clause G13: Foul Water). Your local council or a licensed drainlayer can check this with a CCTV drain inspection.
Preventing the Smell Coming Back
| Cause | Prevention |
|---|---|
| Dry P-traps | Run unused drains monthly; fit trap primers |
| Blocked gully trap | Clean seasonally, especially before winter |
| Blocked roof vent | Annual roof check; fit a vent cowl |
| Root-damaged pipes | CCTV inspection; root treatment or pipe relining |
| Cross-connection | Professional inspection and remediation |
Regular maintenance is the best defence. A quick flush of all your drains before the rainy season (typically autumn in most of NZ) can prevent most odour problems.
When to Call a Professional
Some drain smell issues go beyond DIY. Call a licensed drainlayer or plumber if:
- The smell is accompanied by sewage backing up into the house — this is a health hazard requiring urgent attention.
- You suspect a cross-connection between stormwater and wastewater pipes.
- The problem persists after you've refilled traps and cleaned the gully trap.
- You need a CCTV drain inspection to find cracked, root-damaged, or collapsed pipes.
- Any work involves connecting, disconnecting, or altering drain pipes — this is restricted work under the Plumbers, Gasfitters and Drainlayers Act and must be carried out by a licensed professional registered with the Plumbers Gasfitters & Drainlayers Board.
For complex drainage issues, always check whether a building consent is required through your local council. MBIE Building Performance has guidance on what work requires consent.
You can find qualified, local drain cleaners and drainlayers on tradedirectory.co.nz.
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