A blocked gully trap is one of those unpleasant surprises that can cause sewage or grey water to overflow onto your property, creating a health hazard and a nasty smell. The good news is that clearing a gully trap is often a straightforward DIY job — but it's important to know what you're dealing with before you start, and when the job needs a licensed professional.
What Is a Gully Trap?
A gully trap is a small, ground-level fitting — usually made from PVC or ceramic — found outside the house. It collects wastewater from laundry tubs, kitchen sinks, showers, and floor wastes before it enters the underground drainage system. It has a water seal (like a toilet trap) that prevents sewer gases from escaping into the open air. In New Zealand, gully traps are a standard part of residential drainage systems and are covered under the NZ Building Code, particularly clause G13 (Foul Water).
Signs Your Gully Trap Is Blocked
- Water pooling or overflowing around the gully trap grate
- Slow drainage from sinks, laundry, or showers inside the house
- Unpleasant sewage or sulphur smell near the trap
- Visible debris, grease, or sludge in the trap
Tools and Safety Gear You'll Need
Before you start, gather the following:
- Rubber gloves (heavy-duty, elbow-length if possible)
- Safety glasses
- Old clothes or a waterproof apron
- A garden trowel or gloved hands for scooping debris
- A garden hose with good pressure
- A drain rod set (available from Bunnings or Mitre 10)
- A bucket and rubbish bags
- Disinfectant spray
Safety note: Wastewater contains harmful bacteria and pathogens. Always wear gloves and eye protection, wash your hands thoroughly afterwards, and disinfect any tools and the surrounding area when you're done.
Step-by-Step: How to Clear a Blocked Gully Trap
Step 1 – Lift the Grate
Most gully trap grates simply lift off or unscrew. Remove it carefully and set it aside. You'll likely see standing water or a build-up of debris.
Step 2 – Remove Visible Debris
Using gloved hands or a trowel, scoop out any solid material — hair, food scraps, grease, leaves, or sludge — and place it in a rubbish bag. Don't flush this material further down the drain.
Step 3 – Flush with Water
Use a garden hose to flush the trap with a strong stream of water. This can dislodge softer blockages and help you see whether water is now flowing freely through the outlet pipe.
Step 4 – Use Drain Rods if Needed
If flushing doesn't clear the blockage, the obstruction may be further down the pipe. Assemble your drain rods and feed them into the outlet of the gully trap, rotating clockwise as you push forward. Never rotate anti-clockwise, as this can unscrew the rod joints and leave them stuck in the pipe. Work the rods back and forth to break up the blockage, then flush again with the hose.
Step 5 – Clean and Disinfect
Once water flows freely, rinse the trap thoroughly, replace the grate, and spray the surrounding area with disinfectant. Wash all tools and your gloves before removing them.
Common Causes of Gully Trap Blockages
| Cause | Prevention Tip |
|---|---|
| Grease and fat build-up | Never pour cooking fat down the sink |
| Hair and soap scum | Use a hair catcher on shower wastes |
| Leaves and garden debris | Keep the grate clear and in good condition |
| Tree root intrusion | Have roots cleared by a plumber with a drain camera |
| Collapsed or offset pipe | Requires professional inspection and repair |
What You Can and Cannot Do Legally
In New Zealand, drainage work is regulated. Homeowners are generally permitted to clear blockages and clean accessible fittings like gully traps — this is considered maintenance rather than plumbing work. However, any work that involves cutting into, modifying, or replacing drainage pipes is restricted work under the Plumbers, Gasfitters, and Drainlayers Act 2006, and must be carried out by a licensed drainlayer or plumber registered with the Plumbers, Gasfitters and Drainlayers Board (PGDB).
If your blockage keeps returning, or you suspect a broken or root-invaded pipe, don't attempt to dig up or replace pipework yourself — call a licensed professional.
When to Call a Professional
Contact a licensed plumber or drainlayer if:
- The blockage won't clear with rods and flushing
- Multiple drains inside the house are backing up simultaneously (this suggests a main drain blockage)
- You notice cracked, sunken, or damaged pipework around the trap
- There's evidence of tree root intrusion (you can feel resistance on the rods that won't budge)
- Sewage is overflowing in large volumes or entering the house
- The gully trap itself is cracked or broken and needs replacing
A qualified plumber can use a CCTV drain camera to identify exactly where and what the problem is, saving you time and guesswork. You can find licensed local plumbers and drainlayers on tradedirectory.co.nz.
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