An overflowing gully trap is one of those problems that demands attention fast — raw wastewater pooling near your house is unpleasant, unhygienic, and can cause damage to your foundations or garden. The good news is that many gully trap issues are straightforward to diagnose, and some can be fixed with a bit of elbow grease and the right know-how.
What Is a Gully Trap and What Does It Do?
A gully trap is a small, ground-level drainage fitting — usually a square or round grated box — that collects wastewater from household fixtures like laundry tubs, washing machines, bathroom hand basins, and sometimes kitchen sinks. It connects to your underground drain system and includes a water seal to prevent sewer gases from entering your home. In New Zealand, gully traps are a standard part of residential drainage and are covered under the NZ Building Code (specifically clause G13, Foul Water).
Common Reasons a Gully Trap Overflows
Before you can fix the problem, it helps to understand what's causing it. Here are the most common culprits:
- Blocked gully trap grate or pit — Lint, hair, soap scum, leaves, and debris build up over time and restrict flow.
- Blocked downstream drain — If the pipe leading away from the gully trap is partially or fully blocked, water backs up and overflows at the trap.
- Tree root intrusion — Tree roots are a major cause of blocked drains in NZ, especially in older properties with clay or concrete pipes.
- Collapsed or damaged pipe — Ground movement, age, or heavy vehicles can crush or crack underground pipes.
- Overloaded drain — Running multiple appliances at once (e.g., washing machine and dishwasher) can temporarily overwhelm a slow-draining system.
- Incorrect pipe gradient — If pipes weren't laid to the correct fall, solids settle and cause recurring blockages.
Step-by-Step: Clearing a Blocked Gully Trap Yourself
For a simple blockage at the trap itself, this is a job most homeowners can tackle safely.
What you'll need
- Rubber gloves (heavy-duty)
- Old clothes or a disposable apron
- A stiff brush or old toilet brush
- A garden hose with good pressure
- A bucket
- Drain rods (available from Bunnings or Mitre 10)
The process
- Put on your gloves — wastewater carries bacteria and pathogens. This is non-negotiable.
- Remove the grate — lift it off and set it aside. Scrub it clean with your brush.
- Scoop out debris — use a gloved hand or small trowel to remove any built-up sludge, lint, or leaves from the trap pit.
- Flush with the hose — run water into the trap and watch how quickly it drains. If it drains freely, the blockage was in the trap itself — job done.
- If it still drains slowly, the blockage is further downstream. Use drain rods — screw sections together and push them into the outlet pipe, rotating clockwise to break up the blockage. Never rotate anti-clockwise or the rods may unscrew underground.
- Flush again thoroughly once flow is restored.
- Disinfect the area with a diluted bleach solution and wash your hands well.
Preventing Future Blockages
A little maintenance goes a long way:
- Clean the gully trap grate monthly — especially if you have a washing machine draining into it, as lint is a major contributor.
- Fit a lint filter to your washing machine outlet hose.
- Keep the area around the trap clear of leaves and garden debris.
- Avoid pouring fats, oils, or grease down any drain connected to the system.
- Have your drains CCTV inspected every few years if you have mature trees nearby — catching root intrusion early is far cheaper than dealing with a collapsed pipe.
DIY vs Professional: A Quick Comparison
| Situation | DIY OK? | Call a Pro? |
|---|---|---|
| Debris blocking the grate or trap pit | ✅ Yes | Optional |
| Slow drain, using rods clears it | ✅ Yes | Optional |
| Drain rods won't shift the blockage | ❌ No | ✅ Yes |
| Recurring blockages within weeks | ❌ No | ✅ Yes |
| Suspected tree root intrusion | ❌ No | ✅ Yes |
| Damaged or collapsed pipe | ❌ No | ✅ Yes |
| Sewage smell even after clearing | ❌ No | ✅ Yes |
When to Call a Professional
Some drainage work in New Zealand is restricted work under the Plumbers, Gasfitters, and Drainlayers Act 2006. Any work that involves connecting to, altering, or repairing underground drain pipes must be carried out by — or under the supervision of — a licensed drainlayer registered with the Plumbers, Gasfitters and Drainlayers Board (PGDB). This includes:
- Replacing or relining underground drain pipes
- Clearing blockages using high-pressure water jetting equipment (professionally)
- Repairing or repositioning a gully trap fitting itself
- Any work that requires a building consent
If your gully trap overflows repeatedly, or your drain rods hit an immovable blockage, it's time to call in a licensed drainlayer. A CCTV drain inspection is often the smartest first step — it takes the guesswork out of the problem and means you only pay for the repairs actually needed.
You can find qualified, local drain cleaners and licensed drainlayers in your area on tradedirectory.co.nz.
Safety note: Never enter or reach deeply into a drain chamber without proper confined space training and equipment. Sewer gases including hydrogen sulphide can be dangerous. If you smell rotten eggs strongly near a drain, ventilate the area and keep people away. Contact a professional.