The warnings follow heavy rain across southeast Queensland overnight and are clustered around the Brisbane, Gold Coast, Logan and Ipswich council areas.
Residents have been told to move to higher ground away from the water or get ready to leave immediately if the threat worsens.
"If you are near flood waters get up as high as you can where you are," Queensland Police said on Monday.
In northern NSW, where one person has died, rain is falling, roads remain closed and people are warned not to drive through high water.Â
Some 1800 people have been isolated by the deluge and other 18,500 have been told they could be isolated.
Across the northeast, 20,300 people have been told to evacuate.
Over the weekend, some residents were allowed to return to their homes in places such as Lismore as conditions eased.
"We're not out of the woods just yet," NSW SES Assistant Commissioner Sean Kearns said on Monday
"The predicted risk of heavy rain to continue throughout Monday and possibly into Tuesday will only exacerbate this risk."
When Alfred made landfall in Queensland on Saturday, Malcolm Drysdale compared it to being "flogged quietly by wet lettuce".
However, as he trudged around rising floodwaters, Mr Drysdale knew the weather system packed a mighty punch.
But Mr Drysdale is relatively unfazed by flooding after relocating to Rocklea, a suburb in Brisbane's south notorious for inundation.
His family hails from an area outside Lismore, in NSW, which went through devastating flooding in 2022.
"My mum went to Richmond River High School, which no longer exists because the river took it to pieces in 2022," Mr Drysdale told AAP.
"I am no stranger to flooding, nor is anyone in my family. People around here in Rocklea are the same."
Mr Drysdale bunkered down on Saturday when Alfred crossed the coast.
"Wind-wise it was a bit of a non-event, a bit like being flogged quietly by wet lettuce," he said.
But winds picked up again the next night before more wild weather triggered widespread flooding.
"There's a few more trees down than I first thought," Mr Drysdale said as he waded through high water.
"It's always going to flood here ... but it does get a bit hairy at times."
Prime Minister Anthony Albanese is in Lismore on Monday and is expected to hold a press conference.
He flew into Brisbane on Sunday as flooding shut down a Queensland community and falling trees destroyed homes and cars.
Residents at Hervey Bay north of Brisbane were told to stay inside under an emergency declaration after almost 250mm of rain fell in hours, triggering flash flooding.
Parts of Gold Coast received an emergency warning late on Sunday, with communities told to "take action now" and stay off the roads.
About 30 rescues took place across Queensland's southeast and northern NSW on the weekend.
But some were not so fortunate. 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.
Defence Minister Richard Marles said the circumstances of the accident are still being investigated.
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