People may have perished in floodwaters in NSW overnight, the SES fears, as the northern city of Lismore goes underwater.
Flood waters are tearing through Lismore, flooding homes and businesses, as water flows over the top of the city's protective levee, which was raised after previous floods.
The city's siren was activated overnight warning residents to leave immediately.
Flood waters increasing in Chinderah
"It's the first time it's been activated in 12 years," Lismore mayor Isaac Smith told AAP on Friday.
Floodwaters topped the 10.6-metre level at 4.15am on Friday, with water levels predicted to reach 11.5 metres later in the day, Mr Smith said.
Murwillumbah, Chinderah and Kyogle are also expected to be affected by rising waters - more than 19,000 people across the state are subject to evacuation orders.
More than 6000 people have left their homes in northern NSW, as rivers rage with the rain dumped by ex-Tropical Cyclone Debbie, which pummelled Queensland earlier this week.
NSW SES Deputy Commissioner Mark Morrow says there were 130 flood rescues in the NSW area overnight, and fears some who couldn't be reached may have died.
"There could be people overnight that perished in that flood, we don't know at this stage," he told ABC television on Friday.
"We expect this morning that as we start to go out and try to find people that made those calls overnight, there could be some very distressing news."
Video: Floodwaters in northern NSW
Mr Morrow said metres of water would swamp Lismore CBD businesses, and homes were also likely to go under. Flood levels in Lismore could go beyond what the city saw in 2001 and 2005.
"What that means to people listening this morning is that most of the businesses in the Lismore CBD will have up to three metres of water over the floor," he said.
"That's up to about roof level in their house or even higher. A lot of people that are going to be displaced from their businesses or their homes."
Early Morning: Lismore CBD filling up with water
— Jackson Vernon (@jacksonvernon) March 30, 2017
Mr Morrow painted a dramatic picture of efforts overnight to save people from floodwaters in the north.
"About 130 flood rescues, people in cars, people trapped on roofs of houses. We'll get out there as soon as we can in daylight with aerial assets as well, helicopters, and we'll try and find those people that made calls to us last night and help them this morning."
He said about 6000 people had left their homes, with many holed up in evacuation centres in the region.
River levels in some threatened communities are continuing to rise, even on a low tide, when they should be starting to drop, Queensland Fire and Emergency Services Deputy Commissioner Mike Wassing said.
"In some places further to the north, like Chinderah up around Tweed Heads where the water should be falling on a low tide now it's actually increasing. That's not a good sign for today."
The wind is expected to pick up on Friday, compounding the dangers.
"We're far from out of this and I think the recovery efforts that are going to have to occur are going to be significant," Mr Wassing said.
He said 13 evacuation orders remained current, affecting about 25,000 people.
"I can't stress enough for those people - don't wait for the advice of the emergency services ... you can act now and get out."
The Lismore mayor said people had been caught off guard, and the city is in uncharted territory.
Mr Smith said the city's protective levee had never failed Lismore before, but water was now running over the top of it.
"It's never happened, 12 years now the levee's stood there and kept out major and minor floods," he said. Until now.
"A lot of people here are just a bit concerned about how it's going to go, but we honestly don't know."
He said the amount of rain that fell on Thursday hadn't been seen for 30 to 40 years.
"Locals with a bit of knowledge might have seen it coming, but everyone's been caught quite unawares."
Mr Smith said Lismore was a flood town, and locals would be stoic in the face of the latest crisis.
"Anyone who's been there 30-40 years is probably sitting in their house saying: 'It's alright. I've been through this before.'"
Flood waters are expected to peak in Lismore by the early afternoon, and would stay high for about a day, Mr Smith said.
"The water's not going to be going down any time soon, and it will recede very slowly," he said.
The SES has issued Flood Evacuations Orders for the following areas:
- South Murwillumbah, Condong and Tumbulgum.
- Chinderah, Kingscliff and Fingal Head.
- Lismore CBD, and North and South Lismore.
- Tweed Heads South, Tweed Heads West and Tweed Heads.
- Murwillumbah CBD and East.
- Ocean Shores.
- New Brighton.
There is also a flood evacuation warning for downstream of Rocky Creek Dam.
Closer to the Queensland border, residents in 500 homes in the South Murwillumbah, Condong and Tumbulgum areas were also ordered to evacuate, with the Tweed River experiencing major flooding.
People in Tweed Heads South and West, Chinderah, Kingscliff, Fingal Head and Bilambil were told to leave too.
Queensland Fire and Emergency Services Deputy Commissioner Mike Wassing said the state's flood emergency wasn't over, particularly for communities on the NSW border.
"Our greatest area of concern this morning for people living in the areas of Beenleigh, Waterford, Woodend and Beaudesert," he told ABC television.
"We currently got major flood warnings for the Albert River and the Logan River."
He described the situation as "very serious".
"It is very serious, it's a major flood warning. If you're in low-lying areas, safely move to high grounds."
Swift water rescue firefighters had saved the lives of 85 people trapped in floodwaters.
Mr Wassing confirmed there had been "a number of rescues and continue to have as we speak".
At 1am Friday, Queensland police urged residents of Beaudesert to warn neighbours, secure belongings and move to higher ground as the Logan River rose.
Nine News reported some in Beaudesert had to seek refuge on the roof of their cars as the waters rose.
The Bureau of Meteorology said the far northeast of the northern rivers area in NSW had been soaked with 280mm of rain by evening with another 100mm expected by Friday morning.
The dump across the eastern parts of the state is expected to head east once Debbie pulls away from about dawn on Friday, forecaster Mohammed Nabi told AAP.
There'll be heavy rain in the far northeast which will ease during the day, with the chance of thunderstorms.
However, Debbie's retreat will come with strong gale force winds and damaging surf conditions around Yamba, in northern NSW.
Watch: Proserpine after Cyclone Debbie
On Thursday, Prime Minister Malcolm Turnbull toured the cyclone-ravaged areas with Opposition Leader Bill Shorten.
"Nature flings its worst at Australians and it's certainly happened here in the Whitsunday region, but it brings out the best [in people]," Mr Turnbull told reporters.
"The storm's gone, the clean-up will happen and then it's back in business."
The federal government has made money available immediately to the local council to start rebuilding infrastructure - including urgent repairs to roads where damage has cut off towns.
Mr Shorten said if people wanted to help longer-term recovery efforts "the Whitsunday region is a great place for a holiday".
"They will tidy this up, they will be back on their feet," he said.
Sandbags are being collected from six SES depots across the area.
Some flights have been delayed but the Sunshine Coast Airport remains open.