Knowing the difference between your stormwater and sewer drains saves money and helps avoid awkward conversations with plumbers. They are two separate systems with two different jobs.
What stormwater carries
Rainwater from your roof, surface water from your yard. Clean water that runs to the council stormwater system and eventually to a creek or the bay. Not connected to any toilet, sink, or shower.
What the sewer carries
Everything from your toilets, basins, showers, kitchen sink, washing machine, dishwasher. Wastewater that goes to the council sewer system and is treated.
How to tell which one blocked
If your toilet, sink, or shower is slow draining or backing up: sewer issue. Internal fixtures are all on the sewer system.
If water is pooling in your yard, overflowing from a stormwater pit, or coming out of a roof downpipe at the wrong place: stormwater issue.
If you have water coming up through floor drains or your toilet during heavy rain: this can be either. Sometimes stormwater backs up into sewer connections (rare). Usually it is sewer that is full because storm overflow has affected the council main.
Why this matters for pricing
Sewer work is generally more expensive than stormwater. Sewer plumbing is licensed work and requires specific certifications. Most plumbers do both but specialise in one.
Stormwater repairs are often simpler. A blocked pit can be cleared with garden tools. A broken downpipe is a basic repair. Sewer repairs often need CCTV, professional equipment, and licensed work.
Connection to insurance
Most home insurance covers damage from stormwater issues better than damage from sewer backups. Sewer backup coverage is sometimes a separate add-on. Worth checking your policy before you need to claim.
The common confusion
People sometimes describe a 'blocked drain' to a plumber when they have water sitting in the yard. The plumber sets up for sewer work and discovers it is actually stormwater. Time wasted, possibly extra callout cost. When you book, be specific about what is happening: 'toilet won't flush' vs 'water pooling at the back of the property after rain'.
Council responsibility
Both stormwater and sewer become the council's responsibility at the property boundary. If the blockage is in the council main rather than your pipe, they fix it (and you should not be charged). A plumber can usually tell where the problem is by inspection.