In this guide
- First: identify whether your leak is drain-side or supply-side
- The six most common causes of a kitchen sink leak
- Tools and parts you will need
- How to fix a drain-side kitchen sink leak: step by step
- Supply-side leaks: what to do before the plumber arrives
- DIY vs licensed plumber: know the line
- What happens if you leave a kitchen sink leak unrepaired
- Kitchen sink plumbing 101: know your pipes
- Prevention: keep the pipes under your kitchen sink leak-free
- FAQs
A drip under the kitchen sink is easy to ignore when you keep a bucket there and check it every few days. The problem is what is happening to the cabinet base, the subfloor, and the wall cavity behind while you wait. Mould takes 24 to 48 hours to establish in a damp enclosed space. Cabinet particleboard takes longer but swells irreversibly once it is saturated.
Fix the leak now. This guide tells you exactly how, without guessing.
First: Identify Whether Your Leak Is Drain-Side or Supply-Side
This is the single most important step. It determines whether you can legally fix it yourself, what tools you need, and how urgently you need a plumber.
The two-minute isolation test
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1Dry everything first
Open the cupboard under the sink and dry everything thoroughly with a cloth so you can see fresh drips clearly.
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2Turn both taps off fully
Turn off both the hot and cold taps above the sink completely.
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3Watch for 60 seconds with the taps off
If the dripping stops, the leak is on the drain side. If water continues to drip or pool, it is on the supply side.
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4Now run water down the drain
Turn both taps on fully and watch where the drip originates. If it only drips when water is running through the waste pipe, the leak is definitively on the drain side.
| Leak type | What it looks like | Common location | DIY legal in NSW? |
|---|---|---|---|
| Drain-side (waste pipe) | Drips only when water runs down the drain | P-trap, slip joints, basket strainer, waste pipe sections | Yes |
| Supply-side (pressurised) | Drips even with taps fully off | Flexi hoses, isolation valves, copper supply lines | No — licensed plumber required |
NSW law: Under the Plumbing and Drainage Act 2011, any work on a pressurised water supply connection requires a licensed plumber. This covers flexi hoses, isolation valves (the small tap-style valves on the hoses), and copper supply line repairs. Drain-side waste pipe work is not subject to this restriction — it is legal DIY.
The Six Most Common Causes of a Kitchen Sink Leak
Knowing the cause lets you buy the right part before you start. Most hardware repairs under a kitchen sink cost between $3 and $30 in parts from Bunnings.
The plastic nuts connecting sections of drain pipe loosen over years of use and vibration. This is the most common cause and often fixed in seconds — hand-tighten the nut (never use a wrench on plastic slip nuts) and the drip stops immediately.
The flat rubber or nylon washer inside each slip joint compresses and hardens over time, losing its seal. Once cracked or deformed, it leaks at that joint even when the nut is tight. Replace the washer — they cost under $5 and come in multi-packs.
The basket strainer sits in the drain hole of the sink bowl and is sealed with plumber's putty or a rubber gasket. When this seal fails, water leaks from the base of the sink bowl into the cabinet. The strainer itself needs to be removed, cleaned, and resealed.
PVC waste pipes crack under physical impact (a blow from stored items), UV degradation in older installations, or chemical damage from drain cleaners poured repeatedly through the system. A cracked section needs to be replaced, not patched.
The braided stainless flexi hoses connecting your isolation valves to the taps have a typical service life of 10 years. A degraded hose that weeps at a fitting or shows visible bulging in the braiding is a failure waiting to happen — and when they go, they flood the kitchen. Replace immediately.
In homes with older copper supply lines, the pipe wall can develop pinhole corrosion at fittings or elbow joints. Copper touching dissimilar metals or subject to dezincification at brass fittings is particularly vulnerable. This always requires a licensed plumber to repair or section-replace.
Tools and Parts You Will Need
For drain-side repairs, these are the only tools required. Have everything ready before you start — mid-repair trips to the hardware store with water everywhere are avoidable.
Residual water in the P-trap will drain out when you disassemble it.
Inside a kitchen cabinet is dark — do not work blind.
For removing the basket strainer locknut or prying off clips.
For loosening stubborn plastic slip nuts without cracking them.
Buy a multi-pack — they come in various diameters, usually 32mm or 40mm for kitchen sinks.
For any threaded connections you reassemble — 2 to 3 wraps clockwise on the thread.
If resealing the basket strainer — do not use silicone on strainers as it cannot be removed cleanly.
Clean pipe ends and seating surfaces before reassembling — debris under a washer will cause the new seal to leak.
Before you buy parts: Measure the diameter of your drain pipe (most Australian kitchen sinks use 32mm or 40mm waste fittings) and identify the brand of your sink if possible. Caroma, Clark, Oliveri, and Blanco all have slightly different basket strainer designs — take a photo of the existing fitting to the Bunnings counter and staff can match it.
How to Fix a Drain-Side Kitchen Sink Leak: Step by Step
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1Turn off the water and clear the cabinet
Turn off the tap above the sink. For drain-side repairs you do not need to isolate the water supply, but turning off the tap prevents anyone accidentally running water mid-repair. Remove everything from the cabinet beneath the sink so you have clear access and a clear view of all the pipework.
Place your bucket directly under the P-trap. When you loosen the P-trap slip nuts, the water sitting in the U-bend will drain out — usually 300 to 500ml.
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2Locate the exact drip point
Dry all pipes and joints with a cloth. Run water briefly down the sink drain and watch with a torch for where the first drop appears. Mark it mentally — this is your target. Common drip points:
- At the slip joint nuts on the P-trap: washer failure or loose nut
- At the outlet from the sink bowl (the basket strainer): failed putty seal or strainer gasket
- Mid-pipe on a straight section: crack or split in the PVC
- Where the waste pipe connects to the wall outlet: loose connection or failed washer at the wall trap
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3Fix a leaking P-trap or slip joint
This covers the majority of drain-side kitchen sink leaks.
- Try hand-tightening the slip nut at the leaking joint first. Many leaks stop at this point — the nut has simply worked loose over time. Do not use a wrench on plastic slip nuts; you will crack them.
- If tightening does not stop the drip: unscrew the slip nut by hand (or with slip-joint pliers if it is stiff), pull the pipe section free, and look at the washer inside. It will be flat rubber or nylon.
- If the washer is hard, cracked, deformed, or grooved: replace it. Match the diameter exactly. Seat the new washer flat against the shoulder inside the fitting — do not fold or twist it.
- Reassemble, hand-tighten the slip nut, and run water to test. Only tighten further if it still drips — plastic slip nuts crack if over-tightened.
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4Fix a leaking basket strainer
The basket strainer is the chrome fitting in the drain hole of the sink bowl. If water leaks from the base of the sink bowl down into the cabinet (rather than from a pipe connection), the strainer seal has failed.
- From above the sink, remove any rubber seal or plug from the strainer basket. Look at the drain hole — you will see the strainer body sitting in it.
- From below, use slip-joint pliers or a large screwdriver to unscrew the plastic locknut that clamps the strainer to the sink base. Have someone hold the strainer still from above, or wedge it with a screwdriver through the basket holes.
- Once the locknut is off, pull the strainer up and out from above the sink. You will see old plumber's putty or a rubber gasket between the strainer flange and the sink surface.
- Clean all residue from the drain hole and the underside of the sink bowl thoroughly.
- Roll fresh plumber's putty into a rope and press it around the underside of the strainer flange. Alternatively, fit a new rubber gasket if the strainer came with one.
- Press the strainer into the drain hole from above, then refit and tighten the locknut from below. Wipe away excess putty that squeezes out around the strainer flange.
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5Replace a cracked waste pipe section
If you find a visible crack, split, or hole in a section of PVC waste pipe, that section needs to be replaced. It cannot be repaired with tape or putty — the constant water flow will dislodge any patch immediately.
- Note the diameter of the pipe (32mm or 40mm for most kitchen sinks), the length of the section, and whether it uses slip joints or solvent-welded fittings.
- Slip-joint drain pipe (the hand-tightened plastic type) can be replaced directly from Bunnings with no specialist tools.
- Solvent-welded PVC (glued joints, common in older installations) is more involved — if you are not familiar with solvent welding, call a plumber rather than risk a poorly bonded joint that leaks inside the wall.
- When fitting the replacement section: clean both pipe ends, fit new washers at every slip joint, and test before closing the cabinet.
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6Test, then check again after 24 hours
Once reassembled, run hot and cold water for a full two minutes. Watch every joint and connection with a torch. Dry any existing dampness with a cloth so fresh drips are immediately visible.
If there is no drip during the test: leave a dry sheet of paper towel under the P-trap and check it again after 24 hours of normal sink use. Slow drips at imperfectly seated washers can take time to appear. A dry paper towel after a full day of use confirms the repair is solid.
P-trap tip: While you have the P-trap disassembled, clean it out. Kitchen P-traps accumulate grease, food debris, and soap residue. A clogged P-trap increases backpressure on the drain connections, which accelerates washer wear and can force leaks at joints.
Note on pipe sizing: Australian kitchen drain pipes are typically 40mm diameter. Bathroom basins are usually 32mm. They are not interchangeable. Double-check the diameter before purchasing replacement sections.
Supply-Side Leaks: What to Do Before the Plumber Arrives
If your isolation test confirmed a supply-side leak (water drips even with the taps fully off), do not attempt to repair it yourself. Here is how to contain the damage while you wait for SPS.
- Turn off the isolation valve on the affected hose. There are two under your sink — one for hot, one for cold. Turn the valve clockwise (or lever 90 degrees to close). If the valve does not shut the drip off fully, it is also faulty — turn off the mains stopcock instead.
- Find your mains stopcock if the isolation valves do not close. In most homes it is inside the meter box near the front boundary. Turn it clockwise to close. This cuts water to the whole property.
- Place towels and a bucket under the leak and empty the bucket regularly.
- Open the cabinet doors to allow air circulation and reduce the moisture that accelerates mould growth.
- Do not use PTFE tape or silicone as a temporary fix on a pressurised fitting. It will not hold under supply pressure and delays the proper repair.
- Photograph the leak location clearly before anyone touches it — useful for insurance purposes if water has already caused cabinet or floor damage.
DIY vs Licensed Plumber: Know the Line
| Task | DIY legal in NSW? | Typical plumber cost (2026) |
|---|---|---|
| Tightening a loose slip joint nut | Yes | Free (DIY) |
| Replacing P-trap washers or gaskets | Yes | Free (parts under $10) or $150–$250 if plumber called |
| Resealing the basket strainer | Yes | Free (parts under $15) or $150–$300 if plumber called |
| Replacing a section of slip-joint PVC waste pipe | Yes | Free (parts $20–$50) or $200–$350 if plumber called |
| Replacing a flexi hose under the sink | No — licensed plumber | $180–$350 per hose (SPS flat rate) |
| Replacing or repairing an isolation valve | No — licensed plumber | $200–$400 |
| Copper supply line repair or section replacement | No — licensed plumber | $300–$600 depending on access and length |
| Mixer tap replacement (supply side) | No — licensed plumber | $250–$500 supply and fit (SPS flat rate) |
SPS pricing note: SPS Plumbers charges a flat rate per job, not by the hour. Call-out fees are waived for SPS Members. The price quoted before work starts is the price on the invoice — no hourly surprises.
What Happens If You Leave a Kitchen Sink Leak Unrepaired
The drip itself wastes very little water. The damage it causes while dripping is what makes this worth fixing immediately.
| Time left unrepaired | Typical damage | Estimated repair cost (on top of the leak fix) |
|---|---|---|
| 1 to 3 days | Damp cabinet base, possible mould starting on the MDF surface | Nil to minimal — dry it out and monitor |
| 1 to 4 weeks | Swollen or delaminating cabinet base, mould established on surfaces and silicone seals, water staining on cabinet sides | $300 to $800 for cabinet base replacement and mould remediation |
| 1 to 3 months | Subfloor saturation (particleboard swells and becomes structurally unsound), mould spread to adjacent cabinetry and wall cavity | $1,500 to $5,000+ for subfloor replacement, cabinet replacement, and mould remediation |
| 3+ months | Structural subfloor failure, potential wall framing damage, significant mould spread potentially requiring specialist remediation | $5,000 to $15,000+ |
Flexi hose burst risk: A degraded flexi hose under the kitchen sink that is weeping at a fitting can fail catastrophically without warning. A burst flexi hose discharges at full mains pressure — typically 3 to 10 litres per minute. If it bursts while you are at work or asleep, the resulting flood can cause $50,000 to $200,000 in water damage to a home in just a few hours. If your flexi hoses are more than 10 years old or show any braiding corrosion, replace them immediately. Do not wait for them to start dripping. Read our full guide to burst flexi hoses.
Kitchen Sink Plumbing 101: Know Your Pipes
Understanding which pipe does what makes diagnosis faster and repairs more accurate.
| Component | What it does | Material (common) | Leak point |
|---|---|---|---|
| Basket strainer | Sits in the drain hole of the sink bowl; collects food scraps and allows water to pass | Chrome-plated brass or stainless steel | Base of sink bowl (putty seal) |
| Drain tailpiece | Short vertical pipe from the basket strainer to the P-trap | PVC (white or grey) | Top slip joint (basket strainer connection) |
| P-trap | U-shaped pipe that holds water to block sewer gases; the most common leak point | PVC or chrome brass | Both slip joint nuts on either end |
| Waste arm (extension pipe) | Horizontal pipe from P-trap to the wall outlet | PVC | Slip joints at each end |
| Wall trap outlet | The fitting in the wall where the waste arm connects to the drainage system | PVC | Where the waste arm meets the wall — often a rubber compression fitting |
| Flexi hoses | Braided stainless hoses connecting isolation valves to tap inlets — pressurised supply side | Stainless braided rubber | At either end fitting (burst failure is catastrophic) |
| Isolation valves | Shut-off valves on the hot and cold supply lines — pressurised supply side | Brass with plastic handle | Valve body or packing nut (drip around the stem) |
Prevention: Keep the Pipes Under Your Kitchen Sink Leak-Free
Most kitchen sink leaks are predictable. A few simple habits eliminate the majority of them.
- Check under the sink every three months. Open the cabinet, look at every joint, and feel the pipe connections for dampness. A slow weep caught at the inspection costs nothing to fix. Caught after three months of running, it costs hundreds.
- Never pour grease or cooking fat down the kitchen drain. Grease solidifies inside the P-trap and waste pipes, reducing flow and building backpressure on every joint. It is the primary cause of P-trap connection failure in kitchen sinks.
- Use a basket strainer insert to keep food scraps out of the drain. Food debris accelerates deterioration of rubber washers and creates organic growth inside the P-trap that attacks pipe materials over time.
- Do not store heavy items on top of exposed drain pipes. Pots, cleaning products, and bin liners piled against the P-trap apply physical stress to slip joints and can crack them.
- Replace flexi hoses every 10 years regardless of visible condition. The internal rubber degrades before the braiding shows external signs. If your hoses are original to the house and the house is over 10 years old, replace them now.
- Install a water leak detector under the sink. Small wireless sensors that alert your phone cost under $30 and detect water pooling within seconds of it starting. Highly effective for catching supply-side failures before they become floods.
- Avoid chemical drain cleaners. Caustic drain cleaners damage rubber washers and seals, degrade older PVC fittings, and can warp chrome P-traps. Use a drain snake for blockages instead, or call SPS.
