A leaking compression fitting under the sink is one of the more common plumbing headaches for NZ homeowners — and the good news is that it's often something you can sort yourself without calling a plumber. That said, it's important to understand what's causing the leak before you start tightening things, because the wrong approach can make matters worse.
What Is a Compression Fitting?
A compression fitting joins two sections of pipe (or a pipe to a valve or tap) without soldering or gluing. It works by compressing a small brass or plastic ring called an olive (or ferrule) against the pipe as you tighten the nut, creating a watertight seal. You'll commonly find them on the supply lines running to your sink mixer tap, or on isolating valves under the basin.
Why Is It Leaking?
Before reaching for a spanner, identify the source. Common causes include:
- Loose compression nut — the most frequent culprit, often from vibration or a fitting that was never fully tightened
- Damaged or deformed olive — if the fitting has been over-tightened or disturbed, the olive may no longer seal properly
- Scored or damaged pipe — a nick or scratch on the pipe surface prevents a good seal
- Incompatible materials — mixing plastic and copper pipes with the wrong fittings can cause leaks
- Old or brittle components — fittings degrade over time, especially in older NZ homes with original pipework
Tools and Materials You'll Need
- Adjustable spanner or basin wrench
- Slip-joint pliers
- Clean cloths or a small bucket
- Replacement olive (brass is preferred over plastic for durability)
- PTFE tape (thread seal tape)
- Replacement compression fitting or supply line (if needed)
You can pick up olives, PTFE tape, and replacement fittings at Bunnings, Mitre 10, or PlaceMakers.
Step-by-Step: How to Fix the Leak
1. Turn Off the Water Supply
Always start here. Turn off the isolating valve on the supply line under the sink (slot-head screw, turn 90°). If there's no isolating valve, turn off the mains at the toby (usually near the boundary or water meter). Open the tap above to release pressure and drain any remaining water.
2. Dry the Area and Confirm the Leak Point
Wipe everything dry with a cloth, then run your finger around the fitting to pinpoint exactly where the water is coming from — the nut, the olive area, or the pipe itself.
3. Try Tightening the Compression Nut First
Using an adjustable spanner, give the compression nut a quarter-turn clockwise. Don't over-tighten — this is a very common mistake that crushes the olive and makes the leak worse. Turn the water back on briefly and check. If it's stopped, you're done.
4. If Still Leaking — Disassemble and Inspect
Turn the water off again. Unscrew the compression nut fully and slide it back along the pipe. Inspect the olive: if it's cracked, deformed, or heavily scored, it needs replacing. Also check the pipe end for scratches or damage.
5. Replace the Olive and Reassemble
Slide the old olive off (you may need to carefully cut it off with a hacksaw if it's stuck — take care not to score the pipe). Slide on a new olive of the correct size (typically 15 mm for standard NZ household supply lines). Reassemble the fitting hand-tight, then tighten with a spanner — one to one-and-a-quarter turns past hand-tight is the general rule for a new olive.
6. Test for Leaks
Slowly turn the water supply back on and watch the fitting closely for 60 seconds. Dry the area first so any new drip is immediately visible.
A Note on PTFE Tape
PTFE tape is used on threaded fittings, not on the olive/compression part of a compression fitting. If your fitting has a threaded connection (e.g., into a valve body), wrapping 3–4 layers of PTFE tape clockwise on the male thread before reassembly can help stop weeping at that joint.
Flexible Supply Lines
If the leak is coming from a braided flexible supply line (the hose connecting the tap to the valve), it's often easier and safer to simply replace the whole hose rather than try to repair it. These are inexpensive and available at most hardware stores. Check the thread size before you go (commonly 3/8" or 1/2" BSP in NZ).
When to Call a Professional
Some situations are best left to a licensed plumber. Under the Plumbers, Gasfitters and Drainlayers Act 2006, certain plumbing work in NZ is restricted work that must be carried out or supervised by a licensed tradesperson. While homeowners can generally do minor maintenance like tightening fittings or replacing tap washers on their own fixtures, any work that involves:
- Cutting into or modifying existing pipework
- Installing new valves or fittings on the mains supply
- Repeated or unexplained leaks (which may signal deeper issues like water hammer or pipe corrosion)
- Leaks near electrical wiring or appliances
…should be handled by a professional. Water damage can also affect your home's compliance with the NZ Building Code, particularly clauses E3 (internal moisture) and G12 (water supplies).
If you're not confident, or the leak keeps coming back, it's worth getting a qualified local plumber to take a look. You can find licensed plumbers in your area on tradedirectory.co.nz.
Sources & Further Reading
- Plumbers, Gasfitters and Drainlayers Board (PGDB) — licensing and restricted work
- MBIE Building Performance — NZ Building Code and compliance
- BRANZ — building research and best practice guidance
- Bunnings NZ — plumbing fittings and tools
- Mitre 10 NZ — hardware and plumbing supplies
- WorkSafe NZ — safety guidance for home maintenance