A large blaze has been contained in Adelaide, South Australia after it burned through 2,500 hectares.
Neighbouring Victoria state is facing its worst fire risk in a year.
Temperatures in those states have started to cool but New South Wales and Queensland will see their heatwave continue into Tuesday.
Sydney recorded temperatures of above 40C by Monday afternoon.
Health officials have urged people to stay inside and to avoid physical activity, and for those near bushfires to avoid inhaling smoke.