A dripping pipe joint under the kitchen or bathroom sink is one of the most common household plumbing issues in New Zealand. The good news is that if the leak is on the waste (drain) side, many repairs are well within a confident homeowner's ability. However, if the leak is on the supply (pressurised water) side, that's a different story — and in many cases, it's legally restricted work requiring a licensed plumber.
First: Identify Which Side Is Leaking
Before grabbing a spanner, work out exactly where the leak is coming from:
- Waste pipes (drain side): These are the larger plastic pipes — usually white or grey — that carry water away from the sink. They are not under pressure and are generally DIY-friendly to repair.
- Supply pipes (hot and cold water side): These are the smaller pipes or flexible braided hoses connected to your tap. They are under mains pressure. Repairs to these connections may constitute restricted plumbing work under the Plumbers, Gasfitters and Drainlayers Act 2006, meaning only a licensed tradesperson can legally carry out the work.
If you're unsure which side is leaking, dry everything off with a cloth, then run the tap and watch carefully to see where the drip originates.
Tools and Materials You'll Need
Fixing a Leaking Waste Pipe Joint (DIY)
Most under-sink waste systems use push-fit or threaded plastic fittings with rubber washers or O-rings. Here's the general process:
Step 1 — Clear the cabinet and protect the floor
Remove everything from under the sink and lay down old towels. Place a bucket under the leaking joint to catch any water.
Step 2 — Turn off the water supply
Turn off the tap above, or isolate the supply at the isolating valves (the small valves on the supply pipes under the sink). Run the tap briefly to release any pressure.
Step 3 — Disassemble the leaking joint
Unscrew the slip nut (the large plastic nut) by hand or with slip-joint pliers. Be gentle — overtightening plastic fittings can crack them. Pull the fitting apart and inspect the washer or O-ring inside.
Step 4 — Replace the washer or O-ring
If the washer looks flattened, cracked, or deformed, that's almost certainly your culprit. Take the old washer to your hardware store to match the size. Lightly coat the new washer with silicone grease before reassembling.
Step 5 — Reassemble and test
Reassemble the joint, tightening the slip nut firmly by hand, then just a quarter-turn more with pliers. Do not overtighten — this is a common cause of cracked plastic fittings. Run the tap and check for leaks.
Fixing a Leaking Trap
The P-trap (the curved section that holds water to block drain gases) is another common leak point. If the trap itself is cracked or the joints are weeping, you can replace the entire trap assembly. Replacement traps are inexpensive and widely available. Make sure you buy the correct diameter — typically 32 mm or 40 mm for NZ sinks.
Using PTFE Tape on Threaded Joints
For any threaded plastic or metal joints on the waste side, wrapping 2–3 layers of PTFE tape clockwise around the male thread before assembly can help create a better seal. This is a standard trade technique and perfectly safe for DIYers to use on waste fittings.
What About Supply-Side Leaks?
If the leak is coming from the flexible braided hoses, the isolating valves, or the connections at the base of the tap, this is pressurised pipework. Under the Plumbers, Gasfitters and Drainlayers Act, work on pressurised water supply systems is restricted work and must be carried out by a licensed plumber. Attempting this yourself could void your home insurance and may be illegal. Check the PGDB website to verify what's restricted and to find a licensed professional.
When to Call a Professional
Call a licensed plumber if:
- The leak is on the pressurised supply side (braided hoses, isolating valves, tap connections)
- The pipe or fitting is corroded, cracked, or damaged beyond a simple washer replacement
- You have older copper or galvanised steel pipework that needs soldering or specialist repair
- The leak has caused water damage to cabinetry or flooring — a plumber can assess the full extent
- You're simply not confident — there's no shame in calling in an expert
A qualified local plumber can diagnose and fix the problem quickly and safely. You can find licensed plumbers in your area on tradedirectory.co.nz.
Safety Note
Always turn off the water supply before starting any plumbing work. If you smell anything unusual or suspect the leak has been ongoing for some time, check for mould or structural damage to cabinetry — prolonged moisture can cause significant hidden damage. WorkSafe NZ has guidance on working safely in confined spaces like under-sink cabinets.