Blocked drains: the five causes, in order
The most useful test in drainage is free: run water at several fixtures and watch what else reacts. It tells you how far down the blockage is before anyone puts a camera in.
Before you buy a drain product or ring anybody, spend one minute establishing how far down the blockage is. Everything else follows from that.
The sixty-second test
Run water at the kitchen sink, the basin, the shower and flush the toilet, and watch what happens elsewhere.
- Only one fixture is slow and nothing else changes → the blockage is local, in that fixture's trap or its branch.
- More than one fixture is affected, or the lowest fixture in the house backs up when you use a higher one, or you hear gurgling in another room → the blockage is in the main line, and everything upstream is queuing behind it.
Gurgling is the giveaway. It is air being pulled through a water seal because the pipe is partly blocked downstream, and it means the problem is not where you are standing.
The five causes at a glance
| Cause | Where | How it shows up | Does clearing end it? |
|---|---|---|---|
| Hair and soap | Local — shower, basin | One fixture gets gradually slower | Yes |
| Fat and food | Local — kitchen line | Recurs further and further from the sink | Temporarily |
| Tree roots | Main line | Seasonal rhythm; gully overflows; several fixtures react | No — needs the pipe repaired |
| Collapsed or displaced pipe | Main line | Blocks repeatedly in the same spot with no obvious cause | No — structural |
| Foreign objects, wipes | Either | Sudden, not gradual | Yes, once removed |
The five causes
1. Hair and soap (showers and basins)
The classic local blockage. It builds slowly, so the fixture gets gradually slower rather than stopping overnight. It sits in the trap or just past it, and it is the one most likely to respond to mechanical removal.
2. Fat, oil and food (kitchen sinks)
Fat goes down warm and liquid and sets on the pipe wall as it cools, narrowing the bore over months. Hot water moves it a little further along and it sets again. This is why kitchen blockages recur further and further from the sink.
3. Tree roots (main line)
The most common serious cause in older Australian suburbs with clay or earthenware pipe and mature street trees. Roots find a joint or a hairline crack, get in, and thrive because the pipe contains exactly what they want. Clearing them is temporary by definition — see why they come back.
4. Collapsed, cracked or displaced pipe (main line)
Ground movement, age, or a section that has dropped and now holds water. The symptom is a main line that blocks repeatedly in the same spot with no obvious cause. This is the one where a camera earns its money, because the fix is structural rather than a clean-out.
5. Foreign objects
Wipes are the big one — including the ones sold as flushable, which do not break down like paper. Also sanitary items, cat litter, and on outdoor drains, silt and leaf litter.
The gully is a warning device, not a fault
The outside gully trap is deliberately the lowest and most accessible overflow point on the property. If the main line blocks, it is designed to spill there — outside, at ground level — rather than through the shower inside the house. Water pooling at the gully is not a gully problem. It is the main line telling you, and it is telling you early.
What actually clears what
- Plunger: good on a local blockage with water sitting above it. Useless on a main line.
- Removing and cleaning the trap: the honest fix for hair, soap and food, if you can get to it and can catch the water.
- Drain snake: reaches further along a branch. On a main line, a hand snake generally does not get there.
- Hydro jetting: high-pressure water, which is what actually cuts fat and root mass out of a main line. Not a DIY tool.
- Drain camera: clears nothing. It tells you why, which is what stops you paying for the same job every year.
When it is not yours to fix
A blockage on your side of the property boundary is generally your responsibility; the sewer main in the street is the water authority's. If you have repeated main-line blockages, or the camera finds a broken or root-invaded section, that is licensed work — clearing, relining or replacing a section of drain is plumbing work under Australian rules. See licensed work.
Frequently asked questions
How do I know if a blockage is in the main line or just one fixture?
Run water at several fixtures. If only one is slow, it is local. If more than one is affected, the lowest fixture backs up, or you hear gurgling in another room, the blockage is in the main line.
Why is water pooling around my outside gully?
That is usually the main line blocked, not the gully. The gully is designed to be the lowest overflow point so a blockage spills outside instead of inside the house.
Are flushable wipes actually flushable?
They pass through the pan but they do not break down like toilet paper, and they are a common cause of blockages further down the line and at pump stations.
Is a drain camera worth paying for?
If the blockage is a one-off local one, no. If the main line has blocked more than once, yes — the camera tells you whether it is roots or a broken pipe, which is the difference between an annual bill and a permanent fix.
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)