Warnings span across southeast Queensland regions including Brisbane, Logan, Gold Coast and Ipswich as multiple swollen rivers and creeks break their banks, rising towards homes.
Ex-tropical cyclone Alfred has battered the southeast with heavy rainfall with some totals exceeding 400mm.
Wanga Wallen on the Gold Coast copped 400mm over the last day, 360mm at West Woombye on the Sunshine Coast and Brisbane City received 300mm.
As a vast region is inundated by a continuous downpour, residents have been told to move to higher ground away from the water or get ready to leave immediately if the threat worsens.
Major flooding is occurring on the Bremer and Albert Rivers and the Warrell and Laidley Creeks in Queensland while NSW's Richmond and Clarence Rivers are also breaking their banks.
There were 17 swift water rescues across Queensland on Monday.
Heavy rain and thunderstorms will continue across southeast Queensland on Monday morning but conditions are expected to ease into the afternoon.
"However with a lot of water to make its way through the river systems, the flooding impacts are likely to continue, certainly into the middle and maybe latter parts of this week," Bureau of Meteorology's Christie Johnson said.
More than 450 schools across Brisbane remain closed on Monday and bus services have been cancelled as roads are inundated by flash flooding.
Hundreds of thousands of homes remain without power but Energex crews are working tirelessly to reconnect the 200,000 properties.
"They are putting everything to try and reconnect people, and it's very difficult to get back on your feet without power," Premier David Crisafulli told ABC Radio.
In northern NSW, some 1800 people have been isolated by the deluge and other 18,500 have been told they could be isolated.
Over the weekend, some residents were allowed to return to their homes in places such as Lismore as conditions eased but authorities are keenly watching further rainfall.
"We're not out of the woods just yet," NSW SES Assistant Commissioner Sean Kearns said on Monday.
When Alfred made landfall in Queensland on Saturday, Malcolm Drysdale compared it to being "flogged quietly by wet lettuce" but while trudging through floodwaters he conceded its impact.
But Mr Drysdale is relatively unfazed by flooding after relocating to Rocklea, a suburb in Brisbane's south notorious for inundation.
"It's always going to flood here ... but it does get a bit hairy at times," he told AAP.
His family hails from an area outside Lismore, in NSW, which went through devastating flooding in 2022.
"I am no stranger to flooding, nor is anyone in my family. People around here in Rocklea are the same," he said.
Prime Minister Anthony Albanese is in Lismore on Monday and is expected to hold a press conference.
Mr Albanese vowed to hold insurance companies to account as southeast Queensland is inundated by water.
"This is a time where they need to do a bit of repair of their relationships with the Australian public by doing the right thing and making payments immediately for people who are eligible," he told the Seven Network on Monday.
The flood disaster turned deadly when the body of a 61-year-old man was found after his ute was swept into floodwaters at Megan, near Dorrigo, NSW, on Friday.
Meanwhile, 13 soldiers were injured after a truck rolled near Lismore on Saturday. One remains in a stable condition in hospital.
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