Thousands have abandoned their homes in northern NSW as the remnants of ex-tropical cyclone Debbie bore down overnight, dumping a staggering amount of rain on the region and causing a major flood emergency in the city of Lismore.
State Emergency Service Deputy Commissioner Mark Morrow said he feared people may have died in the ferocious deluge, which prevented emergency crews from venturing out to rescue those stranded on the roofs of vehicle and houses overnight.
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Torrential flash flooding hits northern NSW
The unprecedented flooding of northern NSW has been documented by hundreds of people on social media as thousands abandoned their homes.
"We couldn't get helicopters in the air, it was too dangerous," Deputy Commissioner Morrow said on Friday morning.
"It was certainly too dangerous in that driving rain to get out there and get to people. So our priority is to try to find whether those people still request our assistance, whether they could get out of the situation that they were in.
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"But we certainly could still see some people [might] have perished in that flooding last night.
"I suspect there may be people that may have perished overnight as a result of those conditions."
The Bureau of Meteorology said that between 500 and 740 millimetres of rain had fallen in the middle and upper reaches of the Tweed River Valley in the 24 hours to 2am on Friday.
That figure is more than double the monthly March average of 217.6mm for the town of Murwillumbah, where residents were ordered to evacuate the CBD and the east of the town in the early hours of Friday.
The highest March rainfall ever recorded in Murwillumbah was 764.4mm in 1978.
Lismore's main streets under water
Further south in Lismore, the State Emergency Service said the situation was "very, very serious" as the levee designed to protect the city cracked and was breached by the floods about 4am on Friday.
An emergency siren sounded throughout the CBD at that time, and residents were ordered to evacuate immediately.
Up to 576 millimetres of rain fell over the Wilsons River valley during the 36 hours to 4am on Friday, the bureau said. The Wilsons River passes through Lismore, and many of the city's main streets are under water.
"Major flooding is occurring along the Wilsons River at Lismore, with river levels predicted to be higher than the 2001 and 2005 floods," the bureau said.
"Depending on the forecast rainfall, further rises are possible."
The river is expected to peak about 11.8 metres on Friday afternoon, Stephanie Spackman, a duty forecaster at the Bureau of Meteorology, said.
The peak is "more likely in the early afternoon", she said.
Key to the easing conditions is the fact the worst of the rain has moved out into the Tasman.
The Wilsons Valley may receive only another 5-10 millimetres of rain on Friday, with showers likely to be a feature of Friday's weather along much of the NSW coast, including Sydney.
The Wilsons Valley has had several big rain events in March, with more than 400 millimetres recorded at Lismore Airport even before this week's deluge.
As a result, most of the rain over the past couple of days would have run straight into the creeks and rivers in the area, Ms Spackman said.
The Wilson River looks like it hasn't finished rising at Lismore: @BOM_NSW pic.twitter.com/vGo4tmqT57
— Peter Hannam (@p_hannam) March 30, 2017
#Lismore quickly slipping underwater. Dozens of businesses flooded, water is rising at a rapid pace @abcnewsSydney @abcnews #NSWFloods pic.twitter.com/dRKoHZKUp1
— Jackson Vernon (@jacksonvernon) March 30, 2017
"For your own safety, you must evacuate now. Move to higher ground away from the floodwaters," he said.
"Rivers will continue to rise and more communities may be impacted.
"I can't stress enough the importance of staying out of the floodwaters. They are extremely hazardous, often fast flowing, and may cover hidden voids in the roadway and contain snakes and sewage. Do not enter floodwaters."
Several Flood Warnings are current, affecting rivers including the Tweed, Wilsons, Orara, Upper Macintyre, Bogan, Bellinger, Brunswick and Marshalls creek.
Evacuation warnings are current for South Murwillumbah, Condong, Tumbulgum, Chinderah, Kingscliff, Fingal Head, Lismore CBD, North and South Lismore, Tweed Heads South, Tweed Heads West, Tweed Heads, Murwillumbah CBD and East, Kyogle, Billinudgel, Ocean Shores and New Brighton.
The rain has been caused by the remnants of ex-tropical cyclone Debbie clashing with a cold front.
Debbie's slow retreat from Friday morning is expected to bring some mild relief from the deluge across the state's east throughout the day.
From sandbags to inundation. This #Murwillumbah woman shot her shop progressively being flooded last night #bigwet. pic.twitter.com/wI8P1ZfvUi
— Siobhan Heanue (@siobhanheanue) March 30, 2017
Tom Hough, a meteorologist with Weatherzone, said showers and windy conditions were forecast for the Northern Rivers region on Friday.
"Any rainfall should be fairly light," he said, adding that less than one millimetre was forecast to fall on that area.
Mr Hough said the worst of the rain had passed in Sydney. The city received 42 millimetres of rain between 9am on Thursday and 7am on Friday.
A further one to five millimetres is forecast on Friday, with a top of 23 degrees.
"For Sydney, the worst of the rain was [on Thursday], but there will be a few showers around throughout the day," he said.
Weatherzone is owned by Fairfax Media, publisher of this website.
with AAP