About 5,000 residents in Lismore, in northern New South Wales (NSW) state, were evacuated from their homes as floodwaters, in some places more than 10 meters (32 feet) deep, surged across riverlands stretching along 300 km (185 miles) of coastline.
Authorities also urged 9,000 people to evacuate the nearby town of Grafton, with floodwaters up to 8 meters deep expected to hit the town late Friday. State authorities declared disaster zones in six areas, unlocking government help.
The storms left 16,000 people without power as fallen trees brought down lines and blocked roads.
Further south near Coffs Harbour, the Bellinger River was expected to peak at more than 8 meters, with more than 3,000 people isolated in the towns of Thora, Darkwood and Bellingen.
In southeast Queensland, hundreds of roads remained cut on Friday, with homes flooded and industrial suburbs in popular tourist beach areas transformed into canals.