Preparations are underway in Rockhampton for the biggest flood in more than 60 years, with army personnel being deployed to assist residents in the central Queensland city.
Forecasters expect a major flood peak to hit Rockhampton by the early hours of Thursday, reaching a level larger than the 2011 floods and potentially matching the February 1954 flood level of 9.4 metres.
"At that level, we would see around 5400 properties impacted including 3,000 residential, 1500 commercial and potentially another 900 properties," Mayor Margaret Strelow said.
It's understood some local supermarket shelves have been left bare as residents scramble to collect supplies ahead of the floodwaters.
The Rockhampton Airport is also expected to close at midday on Monday.
At 9am on Sunday, the main flood peak in the Fitzroy system was in the Mackenzie River downstream of Coolmaringa and predicted to move downstream over the next week.
Army personnel are among the emergency services to have been pre-deployed to the central Queensland city ahead of the floodwaters' spread, with doorknocking also underway.
"This is not something that will suddenly happen, but the water will gradually rise in that Rockhampton and surrounding areas," Police Commissioner Ian Stewart said on Sunday.
Of the state's destructive weather, he warned: "This is not over yet."
Volunteers are frantically working to save sporting venues in the flood zones including the Rockhampton Hockey Association, which was due to relocate to higher ground at the end of 2017.
President Barbara Knowles said any damage to the current grounds would be taking money away from the crucial relocation fund.
"I couldn't bear the thought of having to spend another $500,000 or more on cleaning this up," Mrs Knowles told AAP.
The Bureau of Meteorology has issued major flood warnings for the Connors, Isaac, Mackenzie and Fitzroy Rivers.
The preparations come after the body of Eagleby man Nelson Raebel was found by emergency crews on Saturday afternoon, becoming the Queensland's first victim following ex-tropical Cyclone Debbie.
Authorities hold grave concerns for three others still missing: a man in his 60s who went bushwalking at Lamington National Park, 50-year-old Mondure man David Heidemann and 58-year-old John Frost from Mount Pleasant in Mackay.
A submerged vehicle has also been found at Kooralbyn, with police seeking more information given no missing persons report has been filed.
Floodwaters are expected to start to recede in parts of Queensland's southeast, with about 30 homes in the Albert River catchment and 200 in the Logan River area likely to have been damaged.
Hundreds are also without power in the region - including suburbs such as Beenleigh, Eagleby and Loganlea - after Energex cut supply to stem the threat of electrocution.
Queensland Fire and Emergency Services commissioner Katarina Carroll said 4000 assessments had been undertaken throughout northern parts of the state since Cyclone Debbie hit the coast.
Of those, 650 residences have been ruled uninhabitable, she said.
Rising death toll
Police confirmed the body of a 77-year-old man who went missing in Eagleby was located, raising the death toll from the flooding crisis across New South Wales and Queensland to three people.
On Friday, police discovered 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 on Friday.
NSW Police also said the body of a 45-year-old man was located at a caravan park in south Murwillumbah on Saturday, although it is yet to be confirmed that the death is related to the flooding.
Police 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 and Stewart urged residents in low-lying areas to leave.
"By Wednesday, we will be at peak flooding in Rockhampton," he said.
"It will be a gradual rise, so I encourage people to move now."
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, cutting roads.
Watch: Floodwaters near Murwillumbah
Flood-affected houses still unsafe: NSW SES
The SES is warning northern NSW residents not to enter their potentially unsafe, flood-ravaged homes and businesses as waters recede.
At Lismore, the water level has dropped by five metres after peaking within a metre of the 1974 record of 12.2m on Friday afternoon.
Deputy acting commissioner Mark Morrow said entering damaged properties in the CBD posed a safety risk.
He said it could be hours until an evacuation order was lifted.
"We're dealing with buildings that potentially could collapse," Mr Morrow told the ABC.
"We're dealing with sanitary issues, we're dealing with water issues."
People on the fringe of the flooding began cleaning up on Saturday as large parts of the town remained under water.
The rural fire service will set up a base camp in the area for 350 emergency service workers to help with the clean-up effort, Mr Morrow said.
A deluge caused by ex-Cyclone Debbie caused flooding across large parts of the region.
The small indigenous community of Cabbage Tree Island was evacuated as the water rushed downstream on Saturday.
Thousands of sandbags have been used at Ballina but it is now thought residents will not have to leave town.
Roads around the town could be closed due to inundation of low-lying areas, Mr Morrow said.
Lismore mayor Isaac Smith said the town was still contending with a moderate flood level and up to 60 businesses still had water in them.
Those who returned to begin the clean-up spent Sunday morning piling damaged stock and ruined equipment outside.
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"To be honest, it's like a war zone," Mr Smith told AAP.
"There is just so much debris floating around. It's really hard to even assess how long it's going to take to collect all of this rubbish."
Two women, aged 36 and 64, died after being caught in fast-moving waters, prompting Prime Minister Malcolm Turnbull to again warn communities to take care.
A 45-year-old man's body was found at a South Murwillumbah caravan park on Saturday afternoon but it was unclear whether his death was flood-related.
A 46-year-old man also died at his Murwillumbah home but it is understood he had not entered the floods.
Premier Gladys Berejiklian is expected to visit northern NSW on Monday and has pledged state government help for those affected.
