Signs it's more than the sink
- Both bowls of a double sink are slow.
- The dishwasher or washing machine empties back into the sink.
- The gully trap outside fills up when you pull the plug.
- Other fittings in the house are slow or gurgling.
- The same sink blocks again within weeks.
Why sinks and basins block
Fat, oil and grease go down warm and runny, then set as they cool, coating the inside of the pipe. Food scraps and coffee grounds stick to that coating, and the opening shrinks a little more each week. Tauranga City Council puts fats and cooking oils, along with wet wipes, among the causes of most wastewater blockages.
Bathroom basins collect hair, soap and toothpaste in the trap under the plughole. A dropped toothpaste cap or ring can wedge itself there as well.
Where the kitchen waste discharges outside at a gully trap, fat can coat the gully trap too. Our page on outside drains and gully traps covers that, and the main blocked drains page explains how to tell a sink blockage from a drain blockage.
What a plumber will do
Place the blockage
A plumber will check the other bowl, the dishwasher outlet and the gully trap to work out where the water is stopping.
Clean the trap
They'll take off the trap under the sink and clear it, which is where most basin blockages sit.
Clear the waste pipe
A small drain machine goes along the waste pipe towards the gully trap, cutting through fat or food.
Go outside if needed
If the water is stopping past the gully trap, they'll clear the drain from outside, and may suggest hydro jetting for a heavy grease build-up.
What changes the job
- Where it's stuck: a trap is quick, while a fat plug deep in a long waste pipe takes longer.
- Appliances: a dishwasher or waste disposal unit plumbed into the sink adds connections to check.
- Getting under the sink: a packed cupboard or a boxed-in trap slows the start.
- Years of grease, which can need jetting rather than a cable.
What you can do, and what to leave
You can
- Bail out standing water, then plunge with a cloth held tight over the overflow or the second bowl.
- Scrape fat and scraps into the bin rather than the sink.
- Keep a strainer in the plughole and empty it often.
- PGDB's homeowner list includes installing appliances such as dishwashers, and repairing or replacing taps and plugs.
- Clearing a blocked drain is allowed too, as long as you don't touch the inspection pipe or vent pipe.
Leave to an authorised tradie
- Taking the trap off: PGDB's list says homeowners can't fix or unfix traps or waste pipes.
- Machines in the waste pipe or the drain.
- Moving or adding pipes for a new sink, dishwasher or waste disposal unit.
- Anything at or past the gully trap.
Licences and paperwork
Traps, waste pipes and the water pipes feeding a sink are sanitary plumbing, restricted to people PGDB has authorised. Taps and plugs are the exception the law carves out, which is why you can change those yourself.
If the job grows into moving the sink or adding a basin, ask about consent. Schedule 1 exempts altering existing sanitary plumbing when the number of fixtures doesn't go up, no specified system is affected and an authorised person does the work.
Look up the plumber's own name on the PGDB register, and see how to choose a plumber for what else to ask.
Questions to ask the plumber
- Was it the trap, the waste pipe or the drain outside?
- Is there more fat further along that will cause trouble later?
- Does the dishwasher connection need changing?
- What would you change to stop it happening again?
What we would do
Bail the water out and give the plunger a proper try, with the overflow covered. If the sink still holds water, or the gully trap rises when you drain it, leave the trap where it is and book a plumber. Then fix the habit behind it: fat in the bin, a strainer in the plughole, and nothing but water down the sink.