At least two people are dead and about 20,000 people have been ordered to evacuate homes in northern NSW while in southeast Queensland, more than 300 properties were at risk of being swamped by "unprecedented" floodwaters.
The tail end of cyclone Debbie has brought unprecendented rainfalls, flooding wide swathes of the region.
Low-lying areas of the Brisbane metropolitan area of Logan is expected to be among the worst affected as the Albert and Logan Rivers swell to never-before-seen levels.
"The mayor will tell you that we've never seen the heights of the floodwaters down here before ," Queensland Premier Annastacia Palaszczuk told reporters in Beenleigh.
The area's much-loved Yatala Pie Shop fell victim to the rising water, while the new $15 million John Muntz Bridge was ripped apart by the raging Coomera River.
In the NSW Hunter region, a 64-year-old woman died after her and her husband's car was swept off a causeway at Gungal, west of Muswellbrook.
Another woman's body was found by a family member on Friday morning after she disappeared overnight in floodwaters at the rural property at Upper Burringbar, 20 kilometres south of Murwillumbah in northern NSW.
Meanwhile further south, the main streets of Lismore were under water.
There are fears more victims will be discovered in the floodwaters, with state emergency volunteers still trying to complete dozens of rescues in northern NSW.
As night fell on Friday, about 1500 residents of the Tweed Shire town of Banora Point were cut off by rising water.
"Some are on the roofs of houses and trying to get to higher ground," an SES spokeswoman told AAP.
"The area is expected to be isolated for the next 24 to 36 hours."
It's the worst flooding in decades, and five local areas have been declared natural disaster zones - Ballina, Byron, Kyogle, Lismore, Richmond Valley and Tweed.
About 628 millimetres of rain has fallen over the Wilsons River valley since Thursday after ex-tropical Cyclone Debbie collided with a cold front from the south.
At Lismore on Friday afternoon the river came within a metre of the record 1974 flood peak, while at Murwillumbah the Tweed River peaked slightly higher than the 1954 flood.
While hundreds of rescues were being carried out, NSW SES acting deputy commissioner Mark Morrow hit out at "disgusting" footage on social media showing people surfing the floodwaters.
"Thinking it is fun ... it is not," Mr Morrow said in Sydney on Friday.
"You can die - it is that serious."
