What is the difference between sewer and stormwater, and why does it matter?

They are two entirely separate networks. Sewer goes to a treatment plant, sized for a fairly steady daily flow. Stormwater goes, usually untreated, to a creek, river or the sea. Mixing them breaks both systems in opposite directions.

What goes wrong

• Roof water into the sewer overloads the network during storms, which causes raw sewage to surcharge out of gully traps and manholes, often into somebody's yard.
• Waste water into stormwater discharges untreated sewage straight into a waterway.
• Both are offences, and water authorities do investigate, including with dye and smoke testing.

Telling them apart on your property

A sewer gully is usually a round grate at ground level near the house with a raised lip, and it should never be paved over or built up. Stormwater pits are typically square grated pits collecting surface water and downpipes. If you are unsure, a plumber can trace them, and a drainage diagram for a connected property can generally be obtained from the water authority.