How to tell a trap blockage from a main-line one in sixty seconds, what each cause looks like, and which ones come back if you only clear them.
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Blocked drains
Where the water backs up, and what else backs up with it, tells you how far down the blockage is — before anyone puts a camera in.
Local blockage or main line? Answer this first
The single most useful diagnosis in drainage is free and takes a minute: run water at several fixtures and watch what else reacts.
If one fixture drains slowly and nothing else changes, the blockage is in that fixture's trap or branch — hair and soap in a shower, fat and food in a kitchen sink. If more than one fixture misbehaves, or the lowest fixture in the house backs up when you use a higher one, or you hear gurgling in a different room, the blockage is in the main line and everything upstream is backing up behind it.
That distinction decides everything else. A local blockage responds to a plunger, a drain snake or removing and cleaning the trap. A main-line blockage does not, and the four things that cause it are tree roots at a pipe joint, collapsed or displaced pipe, fat build-up, and foreign objects. Roots are the most common in older Australian suburbs with clay or earthenware pipes and mature street trees.
Your gully is a warning device. The outside gully trap is deliberately the lowest, most accessible overflow point on the property: if the main line blocks, it is designed to spill there rather than inside the house. Water pooling around the gully is not a gully problem — it is the main line telling you.
What a camera is actually for. A drain camera does not clear anything; it tells you whether you have a root intrusion that will come back every year, or a broken pipe that needs relining or replacing. Clearing a root blockage without ever looking at why it recurs is how people end up paying for the same job annually.
Drains guides
How roots get into a sealed pipe, why clearing alone never ends it, and what relining does differently.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.
- Sydney Water. Water supply, wastewater and stormwater services
- Australian Building Codes Board. National Construction Code — Volume Three (Plumbing Code of Australia)