Have the P-trap cleared, correctly refitted and retested. If the correctly refitted, confirmed-clear trap restores normal drainage, the blockage was in the trap. If the sink remains slow or blocked, the obstruction is probably farther down the fixture branch or a shared drain. Symptoms alone cannot confirm the exact location.
Check the drainage pattern
The P-trap is the curved fitting beneath the sink. The fixture branch carries wastewater from the trap towards a shared sanitary drain.
| What you observe | What it suggests |
|---|---|
| Only one sink is affected | A blockage in that sink’s P-trap or nearby fixture branch is more likely. |
| The sink backs up after little water enters | The restriction may be nearby, although standing water makes this clue unreliable. |
| Another fixture gurgles or backs up when the sink runs | The blockage is likely beyond where the fixture drains connect. |
| Several fixtures are slow or blocked | A blockage in a shared drain is more likely, but it is not necessarily in the main sewer. |
Gurgling, odours and recurring slow drainage can occur with blockages in different locations, so they are not conclusive.
Confirm whether the P-trap is blocked
The practical check is to have the P-trap cleared, correctly refitted and briefly retested. Finding debris in it does not confirm the diagnosis unless normal drainage returns afterwards.
- Check nearby fixtures: If another fixture backs up, stop running water and avoid using connected fixtures.
- Have the trap cleared safely: Disconnecting it can release dirty water, and incorrect refitting can cause leaks. Use a licensed plumber where required or if the fittings are damaged, concealed, seized or unfamiliar.
- Retest briefly: Normal flow indicates that the trap was the active restriction. Little or no improvement points to a blockage farther downstream. Stop immediately if a joint leaks or wastewater backs up.
Avoid contact with backed-up wastewater. If chemical drain cleaner has been added, do not open or plunge the trap; follow the product label and tell the plumber which product was used.
If the trap is clear but the sink remains blocked, several fixtures are affected or wastewater is backing up, contact our Sydney plumbing team for diagnosis. A plumber may use specialised drain-cleaning or camera equipment where appropriate.