Category-four Cyclone Debbie hit northeastern Australia on Tuesday between Bowen and Airlie Beach in Queensland State, ripping up trees, washing boats onto land and causing widespread damage.
Debbie was downgraded to a tropical low as it tracked southeast, but continued to pack damaging gusts and dump huge amounts of rain all the way down the eastern coast to New South Wales (NSW) State, south of Queensland, and Sydney.
Lismore near the NSW coast was hard-hit, with the State Emergency Service (SES) warning of three-meter high flood levels in the town.
"As we start to go out and try to find people that made... calls overnight, there could be some very distressing news," SES Acting Deputy Commissioner Mark Morrow told the Australian Broadcasting Corporation.
"There could be people overnight that perished in that flood, we don't know at this stage."
Other towns subject to evacuation orders include Tweed Heads, Kingscliff, and Murwillumbah.
Further north in Queensland, the popular tourist city of Gold Coast and other nearby regions were also inundated by water. Upper Springbrook in the Gold Coast hinterland recorded 789 mm of water during Thursday, the Bureau of Meteorology said.
Focus has turned to the restoration of essential services such as water and electricity in the areas hit by Debbie, including Bowen, Mackay, and the Whitsundays where some 50,000 people were still without power, officials said.
The military has mobilized 1,300 soldiers to help assess the full extent of damage and aid the clean-up, with helicopters and planes deploying to restore infrastructure and supply emergency food, water, and fuel.