Understanding roughly what happens beyond the fixtures makes several common problems easier to interpret.
Every fixture drains through a trap, which is the bend holding a small volume of water. That water is the seal preventing gas from the drainage system entering the room, which is why a fixture that has not been used for a long time can smell: the trap has evaporated dry.
From there, wastewater joins the property's drain line, which runs by gravity toward the connection point at or near the boundary. Because everything shares that line, a restriction in it affects several fixtures at once, which is the pattern that distinguishes a main line problem from a local one.
Stormwater is a separate system carrying rainwater from roofs and paved areas, and the two are meant to stay separate. Cross connections between them cause problems in both directions.
Beyond the boundary the responsibility generally passes to the water authority, and Sydney Water publishes where that transition sits.
The practical value of knowing this is diagnostic. One slow fixture is local. Several at once is the shared line. Smell without a blockage is usually a trap or a vent. Each points somewhere different.
If water is entering the house faster than it can drain, turn off the mains water, keep clear of any electrical fittings that are wet, and call for help. Do not try to clear a surcharging sewer or stormwater pit yourself, because the water can carry contamination and the pit may be deeper than it appears.
