About 15,000 people have been evacuated on the north coast and the entire Lismore CBD is inundated after days of unrelenting torrential rain.
The Bureau of Meteorology says Lismore's Wilson River could peak at 14 metres on Monday night.
The BOM has issued a severe weather warning for the Northern Rivers and parts of the mid north coast and Northern Tablelands with the potential for life-threatening flash flooding.
Multiple towns across the north have been evacuated as flooding inundates the region, many roads are cut and the State Emergency Service is warning people to heed warnings and avoid entering floodwaters.
Lismore Mayor Steve Krieg says the situation is "very dangerous and life-threatening".
People were caught off guard by the speed of the rising Wilson River, with some people trapped in their homes and on roofs.
The intensity of rainfall the area copped in the last 24 hours left people unprepared, he said.
At 6pm on Sunday the BOM predicted the levee would be breached at midday on Monday but the rain was so intense it happened at 3am.
"It's due to be Lismore's biggest flood in history," he told Sydney radio 2GB.
The SES had been plucking people from floodwaters and Mr Krieg says he took a phone call from a distressed woman whose pregnant daughter was "crying out for help", waiting to be rescued from the roof of her house and unable to contact authorities, he said.
The Richmond River is causing major flooding at Kyogle, Coraki and Bungawalbyn, moderate flooding at Woodburn, and minor flooding at Wiangaree and Casino.
South Murwillumbah has also been evacuated as roads are cut and the only way out is by boat.
The SES ordered the town of Mullumbimby to evacuate on Monday morning as the area is threatened by rapidly rising floodwaters from the Brunswick River.
Fire and Rescue and the SES have been door knocking to ensure people evacuate.
Police will resume a search for a man who went missing in floodwaters in Lismore on Sunday.
Officers heard him calling out for help about 4pm but "lost sight of the man a short time later".
SES volunteers had carried out 70 flood rescues since torrential rain began falling on Tuesday, Premier Dominic Perrottet told reporters on Sunday.
"We know that whilst there might be blue skies in certain parts of NSW that does not mean that there will not be significant flooding events that occur over the course of this week," he said.
One NSW life has already been lost, with a man killed when his LandCruiser was carried away by floodwaters on the Central Coast, north of Sydney, on Friday.
