Queensland police warned that the Logan River, which runs through Beenleigh south of Brisbane, would not hit peak flood levels for several more hours while the city of Rockhampton to the north was also facing a major threat.
Commissioner Ian Stewart warned there was "still a major risk to the community around Logan and further south caused by that flooding situation."
Rockhampton, with a population of over 80,000 on the Fitzroy River, was expected to suffer flood levels not seen for a century by Wednesday and Stewart urged residents in low-lying areas to leave.
"By Wednesday, we will be at peak flooding in Rockhampton," he said.
"It won't just be on Wednesday, it will be a gradual rise, so I encourage people to move now."
Queensland police tweeted "we currently have four people missing... that we have serious concerns about," but provided no further detail.
Tens of thousands of people have been evacuated from a string of towns in Queensland and New South Wales as the floods move south towards Ballina.
Category four Cyclone Debbie hit northeastern Australia on Tuesday between Bowen and Airlie Beach ripping up trees and causing widespread damage that is still being assessed.
It was downgraded to a tropical low as it tracked southeast. But the storm still packed damaging winds and dumped huge amounts of rain all the way down the east coast to Sydney and beyond before blowing out over the Tasman Sea.
Police on Friday found the body of a woman who disappeared in floodwaters near Murwillumbah just south of the Queensland border.
And a 64-year-old woman, whose vehicle was swept off a causeway on a property in Gungal, in the Hunter Valley south of Sydney was also found dead Friday.
Lismore, south of Murwillumbah was among the worst flooded towns on Friday with Tweed Heads, Kingscliff and Murwillumbah also subject to evacuation orders.
Just to the north the popular Queensland tourist city of Gold Coast and surrounds were also inundated.
In areas further north where the cyclone made landfall, water and power were still being restored.
Bowen, Mackay and the Whitsunday islands, where power went down for more than 50,000 people, bore the brunt of the cyclone.
The military has mobilised 1,300 soldiers for the clean-up, with helicopters and planes deploying to restore infrastructure and supply emergency food, water and fuel.
The Insurance Council of Australia declared the Queensland and northern NSW regions disaster zones, adding that the damage bill could top Aus$1 billion (US $770 million).
The council's chief Rob Whelan compared Debbie it to Cyclone Oswald, a category five storm which hit in 2013 and caused over Aus$1 billion in damage.
