There are fears people may have died in "unprecedented" overnight flooding in New South Wales (NSW), while dozens of others remain trapped in cars and on roofs.
The State Emergency Service (SES) received over 1300 calls for help and 148 rescues were carried out overnight as ex-tropical Cyclone Debbie continued to dump heavy rain across the region.
Over 20,000 people have been evacuated from northern NSW and another 20,000 people are subject to an evacuation order.
Sirens have sounded in Lismore, warning residents they must leave immediately.
A torrent of water is rushing through the middle of the Lismore CBD and floodwater has topped the town's levee for the first time since it was built 12 years ago.
SES spokesman Ian Leckie said rescuers had been unable to reach many of those in distress, with 90 flood rescues still outstanding.
"We've got everything happening. We've got people on rooftops, we've got people in vans," he said.
"It's a disaster, an utter disaster. It's probably the fastest event that we've seen in this area for a long, long time. The rate of rise has just literally caught people unaware. People have just been trapped by the floodwaters."
He warned the situation was extremely serious and residents should act with extreme caution.
"When you hear the sirens - that's it, you're out.
"There's no second chance with this one, it's become a highly dangerous situation."
SES acting deputy commissioner Mark Morrow said many businesses and homes in the Lismore CBD would be inundated with up to 3 metres of water after massive amounts of rain overnight.
Evacuation centres had been opened for those who needed assistance.
Donna Walker had to take her children and leave their North Lismore home for one of the centres.
They were making the best of the situation, she said.
"I said to the kids, we're all in the same situation. We're all nervous, we're all upset, we're all not knowing where we're going, what we're doing next, or what's going to happen to our belongings."
Further north, the Logan and Albert River catchments were overflowing after the region received isolated falls of 280mm overnight.
The biggest falls from ex-Cyclone Debbie were recorded in the Gold Coast hinterland.
Upper Springbrook recorded 789mm in the 24 hours to midnight.