A blocked drain connected to a grease trap is one of the more common — and smelly — plumbing headaches in NZ kitchens. The good news is that many blockages on the inlet side of a grease trap can be cleared with basic tools and a bit of patience. However, some work on drainage systems is restricted work under NZ law, so it pays to know where the line is before you start.
Understanding How a Home Grease Trap Works
A grease trap (sometimes called a grease interceptor) is a small chamber installed in the drainage line between your kitchen sink and the main sewer or septic system. Its job is to slow wastewater flow so that fats, oils, and grease (FOG) float to the top and are held back, while cleaner water flows on to the sewer. Over time, the accumulated grease and food solids cause blockages — either in the inlet pipe leading to the trap, inside the trap itself, or in the outlet pipe heading to the sewer.
Understanding which section is blocked will guide your approach.
Tools and Materials You'll Need
- Rubber gloves and safety glasses (grease trap contents are unpleasant and can harbour bacteria)
- Old clothes or a disposable apron
- A bucket and heavy-duty rubbish bags
- A drain plunger (cup-style)
- A hand-operated drain auger (drain snake), 5–10 m
- Hot (not boiling) water
- Dish soap or a purpose-made enzyme-based drain cleaner
- A screwdriver or spanner to open the grease trap lid
- Newspaper or plastic sheeting to protect the floor
Step 1 — Locate and Inspect the Grease Trap
Most residential grease traps are located under the kitchen sink or just outside the house near the kitchen wall. Remove the lid carefully — use a screwdriver if it is screwed down. Do not lean over the open trap without eye protection; gases can be present. Check whether the trap is full of solidified grease or if the blockage appears to be in the inlet or outlet pipe.
Step 2 — Clear the Inlet Pipe First
If water is backing up into the sink but the grease trap itself looks relatively clear, the blockage is likely in the inlet pipe (the pipe from your sink to the trap).
- Use a cup plunger over the sink drain with a firm, steady pumping action — around 10–15 strokes.
- If plunging doesn't shift it, feed a hand drain auger down through the sink drain or directly into the inlet port of the grease trap. Rotate and push gently to break up or retrieve the blockage.
- Flush with hot (not boiling) water once clear. Boiling water can soften PVC pipe joints over time.
Step 3 — Clean Out the Grease Trap
If the trap itself is full or heavily coated:
- Scoop out the accumulated grease and solids into a bucket using an old ladle or scoop. Do not pour grease down any drain — bag it and put it in your general rubbish.
- Wipe down the interior baffles with rags.
- Rinse the trap with warm water and a small amount of dish soap.
- An enzyme-based drain treatment (available at Bunnings or Mitre 10) poured in after cleaning can help break down residual grease and reduce future build-up. Avoid harsh caustic or bleach-based products — these can kill the beneficial bacteria in a septic system and may damage older pipe materials.
Step 4 — Check the Outlet Side
If the trap is clean but drainage is still slow, the blockage may be in the outlet pipe heading toward the sewer or septic tank. You can attempt to snake this pipe from the outlet port of the grease trap. If the blockage is beyond the reach of a standard hand auger (roughly 5–10 m), or if you suspect the issue is in the main drain or sewer line, stop and call a professional.
Preventing Future Blockages
- Scrape plates before washing — keep food scraps out of the sink.
- Pour cooled cooking fats into a container and bin them, never down the drain.
- Clean your grease trap every 1–3 months depending on usage — more often for busy households.
- Run hot water through the sink for a minute after washing greasy dishes.
- Consider a regular enzyme treatment to keep the trap working efficiently.
What You Cannot Do — Restricted Plumbing Work
Under the Plumbers, Gasfitters, and Drainlayers Act 2006, certain drainage work is restricted to licensed drainlayers or plumbers. This includes:
- Altering, replacing, or relocating any part of the drainage system (pipes, traps, connections to the sewer).
- Any work on the drain downstream of the grease trap outlet that connects to the public sewer.
- Installing a new grease trap or modifying an existing one.
As a homeowner, you are generally permitted to clear a blockage using a plunger or auger and to clean out a grease trap — but you must not alter the pipework. If you are unsure, check with the PGDB or your local council. Some councils also have specific requirements around grease trap maintenance and disposal of collected waste, so it is worth a quick call to confirm local rules.
Safety note: If you smell strong sewage gases (hydrogen sulphide has a rotten-egg odour) coming from the trap, ventilate the area immediately, leave the space, and call a licensed tradesperson. Do not use naked flames near an open drain.
When to Call a Professional
Call a licensed drainlayer or drain cleaner if:
- The blockage is beyond the reach of a hand auger.
- Multiple drains in the house are backing up simultaneously (suggesting a main drain blockage).
- You notice sewage odours throughout the house or gurgling from multiple fixtures.
- The grease trap is cracked, damaged, or needs replacing.
- You are on a septic system and the drain field may be affected.
- You are not comfortable working around sewage or confined spaces.
A qualified drain cleaner can use a high-pressure water jetter to thoroughly clear grease-coated pipes — far more effective than a hand auger for heavy build-up. You can find licensed local drain cleaners on tradedirectory.co.nz.
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