Sewer vs Stormwater Blockage: How to Tell the Difference

Two separate systems doing two different jobs

Every Sydney property has two independent underground networks. The sewer line carries wastewater from toilets, sinks, showers and the laundry to Sydney Water's main. The stormwater system carries rain off the roof, down the downpipes, through surface pits and out to the street, a kerb outlet or an absorption pit — it should never carry sewage. Because they are separate, the symptoms are separate too. If a problem appears every time someone flushes or runs water inside, regardless of weather, you are looking at the sewer. If a problem only appears when it rains and shows up outdoors around grates, pits and downpipes, you are looking at stormwater. Knowing which system is involved decides who fixes it, how urgent it is, and roughly what it costs — so it is worth a few minutes to work out before you call anyone.

Sewer blockage: the warning signs

Sewer problems announce themselves indoors and around the lowest fixtures. Watch for toilets that rise then drain slowly, gurgling from the shower or floor waste when the washing machine empties, multiple fixtures backing up at once, and a distinct wastewater or rotten-egg smell. The clearest outdoor sign is your overflow relief gully — a grated drain at ground level outside, usually lower than the lowest indoor fixture, designed to spill outside rather than inside. If it is discharging grey or black water, the sewer is blocked downstream and needs attention quickly. Common causes are tree roots invading old clay joints, wet wipes and sanitary products, congealed fat from the kitchen, and broken or sagging pipe sections. Sewage exposure is a health risk, so an active overflow is genuinely urgent — keep people and pets away from it and stop using water until it is cleared.

Stormwater blockage: the warning signs

Stormwater faults are weather-driven and almost always outdoors. The tell-tale signs are water pooling on the driveway or lawn during rain, surface pits filling and overflowing instead of draining away, downpipes that gurgle or back up and spill at the gutter, and water creeping toward the garage, the house slab or an outdoor power point. After the rain stops, the water usually clears slowly — which confirms it is a flow problem, not a burst. Typical causes are leaves and silt clogging pits and grates, tree roots in the underground stormwater line, collapsed or crushed pipe, and downpipes choked with debris. Stormwater rarely smells. It becomes urgent when water threatens the building, electrical areas or a neighbour's property, because pooling water can undermine slabs and footings. Clearing leaf litter from pits and grates is a sensible first check you can safely do yourself.

How we find it, prove it, then fix it

When the symptoms are ambiguous — or when the same drain keeps blocking — guessing wastes money. Our method is to diagnose and prove the cause before we clear anything. A CCTV drain inspection (guide range $250-$450) puts a camera down the line so you can see exactly what is happening and on which system: roots, a fat build-up, a cracked joint or a collapsed section. Once the cause is confirmed, the right fix follows. Straightforward sewer or stormwater clearing runs $350-$650, with hydro jetting ($300-$550) used to cut roots and scour the pipe wall when a build-up keeps returning. If the camera shows a cracked or root-damaged section, drain relining is quoted after the CCTV so you are not paying for excavation you do not need. Scott Johnstone is licensed in NSW (licence 1-LCV-265), available 24/7across Sydney, and will show you the footage before recommending the work.

Common questions