Check how many fixtures are affected
If one sink drains slowly and everything else is fine, the blockage is local to that fixture. Annoying, not urgent.
If the toilet gurgles when the washing machine drains, or the shower rises when you flush, the blockage is downstream of both. That is the main line, and it affects the whole house.
Look at the overflow gully
Most properties have an overflow relief gully outside, a grated drain set slightly lower than the lowest fixture inside. It exists so that if the sewer backs up, it spills outside rather than into your bathroom.
If that gully is overflowing, the main line is blocked and the system is doing exactly what it was designed to do. Stop using water immediately and call.
Smell and sound
Sewer gas indoors means a trap has been emptied or a vent is blocked. Persistent gurgling from fixtures that are not in use means air is being pulled through the system because water cannot move freely.
Neither is normal, and both point to the main rather than the fixture.
What to do while you wait
Stop running water anywhere in the house. Every load of washing and every shower adds to what has to go somewhere.
Keep people and pets away from any overflow. Sewage is a genuine health risk, not just an unpleasant one. Do not try to clear it with drain chemicals either, because they rarely work on a main line blockage and they make the job hazardous for whoever opens the line.
Written by the team at Adlington Plumbing. Licensed plumbing and gas fitting across Newcastle, Lake Macquarie and the Central Coast. Licence 384065C.