To tell where a leak is coming from under the sink, dry the cabinet, pipes and fittings completely. Place paper towel beneath each connection, test one source at a time and trace the water to the highest point that becomes wet first. Water can travel along a pipe or hose before dripping.
Test the Connections in Order
Check with the sink unused: Inspect the isolation valves, flexible supply hoses and tap connections. Moisture appearing while the tap is off indicates a likely pressurised supply-side leak.
Test the tap connections: Run the tap briefly into a bowl so no water enters the drain. Recheck the hoses and connections beneath the tap.
Test the sink outlet seal: Dry the area, insert the plug and add a small amount of water without reaching the overflow. Moisture directly beneath the outlet indicates a possible drain-seal leak.
Test the waste pipe and trap: Release the plug and watch each waste joint and the trap as the water drains. A leak appearing only during discharge is likely on the drain side.
Check for splash entry: If the plumbing stays dry, apply a small amount of water around the tap base and sink rim. Water below may be entering through an unsealed mounting or rim rather than leaking from a pipe.
Use a torch and paper towel to observe individual joints, but do not dismantle, tighten or disturb fittings.
Interpret What You Find
| When water appears | Likely source |
|---|---|
| While the sink is unused | Isolation valve, flexible hose or supply connection |
| While the tap runs into a bowl | Tap connection or flexible hose |
| While water sits in the plugged basin | Sink outlet fitting or seal |
| While water drains | Waste joint or trap |
| After wetting the tap base or rim | Mounting point or sink-rim seal |
These signs narrow down the source but do not prove which component has failed.
Isolate the Leak Safely
For a supply-side leak, close the relevant isolation valve only if it is accessible and turns normally; never force it. If water is escaping quickly or the valve does not stop it, shut off the property’s main water supply if you can reach it safely.
For a drain-side leak, stop using the sink, place a container beneath the drip and dry the cabinet.
Do not touch wet power points, plugs, appliance leads or waste-disposal equipment. Keep away from the area and arrange urgent help if water is near electrical equipment.
What affects the repair price?
A reliable price cannot be given from the symptom alone. It depends on whether the source is a seal, trap, hose, isolation valve, tap connection or concealed pipe, as well as the scope, access, location, materials, labour, current market conditions and any further faults found during assessment. A licensed plumber should identify the cause and provide a written quote before non-urgent repairs proceed.
Stop testing and contact our Sydney plumbing team if the source remains unclear, water is escaping under pressure or a fitting requires repair. Plumbing repairs that fall within regulated plumbing work must be completed by an appropriately licensed tradesperson in NSW.