Severe weather warnings are in place from Bundaberg in the Wide Bay-Burnett region of Queensland through to Port Macquarie on the NSW Mid North Coast.
In Queensland, a "completely unpredictable" extreme rain event has killed five people, with hundreds evacuated.
The body of a 37-year-old Goomboorian man missing since Friday in floodwaters near Gympie, north of the Sunshine Coast, was found by police divers on Saturday.
The discovery came after a female SES volunteer was killed when a car she was in was swept away en-route to a rescue near Ipswich on Friday night.
In NSW, a man died on the Central Coast on Friday morning after his LandCruiser was carried away in floodwaters.
Queensland's Deputy Police Commissioner Steve Gollschewski urged people not to leave their homes unless they absolutely had to.
"We do not want to lose any more people, so please, please, if it's flooded, forget it," he said.
In Gympie, 700 people living along the swollen Mary River - expected to rise above its highest level in 23 years in the next 24 hours - began evacuating, while supplies were delivered to an Indigenous community in nearby Cherbourg set to be cut off by floodwaters.
Heavy rainfall and potential flash flooding are forecast from Kingaroy through to the NSW border on Sunday.
Intense rainfall is also predicted for an area encompassing Hervey Bay, Yarraman, Toowoomba and Ipswich.
There is major flooding at Tewantin near Noosa, and Picnic Point and Dunethin Rock on the Sunshine Coast.
Major flood warnings are in place for multiple waterways in southeast Queensland.
Brisbane Lord Mayor Adrian Schrinner on Saturday evening issued a flood warning for 16 suburbs along the Brisbane River, saying "several thousand properties" could be inundated.
Residents in low-lying areas of Brisbane city, Bulimba, Hamilton, Milton, New Farm, Newstead, Norman Park, Oxley, Rocklea, South Brisbane, St Lucia, Teneriffe, Toowong, West End, Windsor and Yeronga were urged to be prepared to act.
In NSW, a severe weather alert for Sunday spans 450km from the Queensland border through to Port Macquarie.
There is a risk of "life-threatening flash flooding particularly during Sunday afternoon and evening" across the Northern Rivers and Mid North Coast, in areas including Lismore, Grafton, Coffs Harbour and Dorrigo, said Gabrielle Woodhouse, Senior Meteorologist from the Bureau of Meteorology.
On Sunday and into Monday, the Northern Rivers and Mid North Coast are expected to see "widespread, high rainfall totals" exceeding 100mm, she said.
"This is falling on already wet catchments. What this means is that we're looking at further river rises and a really increased risk of flash flooding."
Late on Saturday, the Insurance Council of Australia declared an insurance catastrophe for southeast Queensland, with claims and assessment teams to begin helping householders as soon as it is safe to do so.
The federal government said disaster assistance had been activated for 10 local councils in Queensland's southeast.
The funds will go to clean-up efforts in the Fraser Coast, Gold Coast, Gympie, Ipswich, Lockyer Valley, Logan, Noosa, North Burnett, Sunshine Coast and Toowoomba LGAs, and will be jointly funded by the federal and state governments.
16°C