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 east coast to New South Wales state, south of Queensland, and Sydney.
NSW Police said the body of a woman who "disappeared in floodwaters overnight from a rural property" near Murwillumbah just south of the Queensland border was found on Friday.
Earlier Friday, SES acting deputy commissioner Mark Morrow said he had fears for some missing residents from Lismore.
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"As we start to go out and try to find people that made... Calls overnight, there could be some very distressing news," he 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.
"I think a lot of people are going to have a lot of significant damage on the farms," he told the ABC.
"It is the worst I have ever seen I have to admit."
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.
"In terms of what happened in the last 24 or 48 hours, we have seen some pretty big rainfall totals," he told Brisbane's Courier Mail.
"There is still that strong risk around the Gold Coast."
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 Whitsunday islands where some 50,000 people were still without power, officials said.