Stormwater and sewerage networks are separate systems, designed differently to meet different needs.
Stormwater networks are managed and maintained by local councils and collect stormwater and direct it, untreated, to local creeks, rivers and waterways. The sewerage network is managed and maintained by Queensland Urban Utilities. It collects all sewage from your house and takes it to a sewage treatment plant for treatment, before it can be reused for commercial purposes or released back into the environment as treated water.
When stormwater enters the sewerage network, sewer pipes (which on average are much smaller in diameter than stormwater pipes) can become overloaded, potentially causing sewage to back-up during heavy rain.
Stormwater pipes should not be connected to the sewerage network.