Stormwater vs Sewer: Understanding Your Home's Drains

Every Sydney home has two separate drainage systems that should never mix: the sewer takes wastewater away for treatment, while stormwater carries rainwater to local waterways. Understanding the difference helps you describe problems to a plumber and understand whose responsibility each pipe is.

What the sewer does

The sewer carries wastewater from toilets, sinks, showers and washing machines to a treatment plant. It's a sealed system, and the section from your home to the property boundary is usually the homeowner's responsibility, while the section beyond the boundary belongs to Sydney Water. Sewer pipes run deep, and problems like tree root intrusion or a collapsed pipe usually show up as slow drains or sewage smells.

What stormwater does

Stormwater carries rainwater from your roof, gutters, downpipes and ground drains to the street and eventually local creeks, rivers or the harbour, usually without treatment. Stormwater pipes are generally shallower than sewer lines, and blockages tend to show up as overflowing gutters, pooling water or flooding during heavy rain.

Why the two must never mix

Connecting the two systems incorrectly causes real problems in both directions. Stormwater flowing into the sewer can overwhelm the sewer during heavy rain and cause sewage overflows, while sewage leaking into stormwater pollutes local waterways. If a plumber finds a crossover connection during a repair or inspection, it needs correcting.

Working out which system is affected

• Sewage smells, or toilets and floor wastes backing up, point to the sewer
• Overflowing gutters, pooling in the yard or flooding in heavy rain point to stormwater
• A gully outside that fills and drains slowly could be either, and a plumber can run a camera to confirm

When to call a plumber

If you're unsure which system is affected, or a drain is backing up into the house, call a plumber rather than guessing. A CCTV inspection can identify which pipe is at fault and what's causing the problem, which saves money compared with clearing the wrong drain.