What to Do When You Find a Leak Under Your Sink

A leak under your kitchen or bathroom sink demands immediate action to prevent costly water damage to your cabinets and flooring. Most commonly caused by loose drain connections, failing supply lines, or worn-out seals, these leaks can quickly escalate from a minor annoyance to a major household emergency. Your first priority is to place a bucket under the leak, then immediately shut off the hot and cold water supply valves located beneath the sink. If you cannot stop the water flow or notice signs of corrosion, contact a licensed plumber straight away.

Within the first two minutes of discovering a leak, you must contain the water, shut off the supply, and clear the area to prevent further damage. These critical steps will stabilise the situation before you identify the source.

1. Contain the Drip – Place a bucket, towels, or a large baking tray directly under the leak. Don’t wait to find the “perfect” container—any water-catching vessel will do. If water is pooling on the cabinet floor, line it with towels immediately.
2. Shut Off The Water – Locate the two small valves on the wall under the sink (one for hot water, one for cold). Turn them clockwise until they stop completely. If there are no valves under your sink, or if they’re not working, you must shut off the main water supply to your house—typically located near your water metre.
3. Clear & Dry the Area – Remove all items from under the sink immediately—cleaning products, bins, and stored items can trap moisture and accelerate damage. Use a sponge or towels to soak up any standing water to prevent cabinet warping and mould growth.

Go to your sink and locate the two shut-off valves right now, even if you don’t have a leak. Familiarise yourself with their location and test if they turn easily. If they’re stuck or corroded, don’t force them, this indicates you’ll need professional help when an emergency arises.

Problem: The shut-off valve won’t turn or is leaking itself.
Solution: Never apply excessive force to a stuck valve. The valve is likely corroded and forcing it could cause it to break completely. Instead, locate your home’s main water shut-off valve (usually near the water metre) and turn it off, then call for professional service.

To find a leak’s source, dry all pipes and connections under the sink thoroughly, then run water to see exactly where the first drop forms. Leaks typically originate from drain lines, supply lines, or the tap base.

Step 1: The Dry Test
With the water off, use a dry paper towel or tea towel to wipe down every pipe, valve, and connection under the sink. This gives you a clean slate to spot new moisture.

Step 2: The Drain Test
Place your bucket back under the P-trap (the U-shaped pipe). Turn the water shut-off valves back on. Plug the sink, fill it with 5-10cm of water, then release it all at once. Watch the drain pipes closely for any drips forming.

Step 3: The Supply Line Test
If the drain test reveals no leak, examine the flexible metal lines that connect the tap to the shut-off valves. Run your fingers along the connections at both the top and bottom, feeling for moisture.

Step 4: The Tap Base Test
Check for water pooling right at the base of the tap on the benchtop. Sometimes water runs down from the tap base and drips underneath, creating the illusion of an under-sink leak.

You’ve successfully found the source when you can consistently see a drop of water forming in the same spot after wiping it dry.

If you’ve identified the P-trap as your leak source and it’s just a loose connection, here’s how to fix it yourself:

1. Position your bucket directly under the P-trap to catch any residual water.
2. Check the slip nuts (the large nuts at each end of the U-bend) with your hand to see if they’re loose.
3. Tighten carefully by turning them clockwise. Hand-tighten first, then give a slight quarter-turn with multigrips or channel-lock pliers if needed. Never overtighten, this can crack plastic fittings.
4. Test your repair by running water through the sink for two full minutes. Place a dry paper towel under the joint to check for any new drips.

Self-Check Method: After tightening, run both hot and cold water through the sink. If the paper towel remains dry after two minutes of continuous flow, your repair is successful.

A plumber typically charges either a service call fee plus hourly rates, or a fixed price for specific repairs. Simple fixes like tightening connections or replacing washers might cost $150-$250, whilst replacing a tap or corroded pipes could range from $300-$800 depending on complexity and parts required.

Call a plumber immediately if you observe:

• Water that won’t stop flowing despite turning off valves
• Visible cracks in pipes
• Extensive corrosion (green or white crusty buildup)
• Water seeping through cabinet walls or flooring
• Any electrical outlets or appliances near the water

Once you’ve resolved your current leak, prevent future emergencies with this quick monthly inspection:

1. Run your hand along all visible pipes feeling for moisture
2. Check for mineral deposits or discolouration around connections
3. Test that shut-off valves turn easily
4. Look for warping or water stains on cabinet floors

A leak under your sink might feel overwhelming, but with these clear steps, you can quickly control the situation and make an informed decision about repairs. Remember: when in doubt, calling a professional plumber is always safer than risking extensive water damage to your home.