Hundreds of people across the region have been left stranded for hours on rooftops amid the crisis as state and federal emergency services struggled to access impacted areas.

"The expectation of the flood waters peaking is now above 16 metres in Lismore and for context the previous record was just a tick over 12 metres in 1954," NSW Premier Dominic Perrottet told ABC Radio on Monday.

He noted that two defence force Blackhawk helicopters were airborne conducting rescues as flying conditions marginally cleared.

The premier described the inundating flood as "unprecedented" and "distressing".

The Lismore CBD is underwater after days of heavy rain that led to the Wilsons River breaching its levee overnight.

Rescuers in a flotilla of dinghies and inflatables plucked stranded residents from the rooftops and balconies of submerged homes on Monday.

Other Lismore residents waded to higher ground carrying belongings and pets.

Susan Raddatz, an acupuncturist in Lismore CBD, climbed out of her first floor apartment window where she was rescued at around 11am by other residents on a motorboat.

"I didn't believe my eyes," she told AAP from a makeshift evacuation centre set up at Goonellabah Sports and Aquatic Centre.

Ms Raddatz said the water levels had by that time risen up to three levels of her building, surprising her with the gushing speed.

"I always thought I'd be safe being on the first floor ... It's never flooded like this. It didn't take much.

She said lots of gas cylinders were afloat in the muddy waters causing concerns for the volunteers about flammability.

The SES has been overwhelmed with over 900 calls for help as the region was hit by its worst rainfall ever, with the situation being described as "dire" and "catastrophic".

There are 17 evacuation orders in place across the state's north while 21 areas are under evacuation warnings, covering around 62,000 people,the premier said.

The crisis is widening, with multiple major flood warnings for northeastern NSW including the Tweed, Richmond, Wilsons, Bellinger and Clarence rivers.

Prime Minister Scott Morrison has offered financial and logistical support to flooded communities, the premier said.

The Bureau of Meteorology expects the far north's heavy rain to ease on Monday afternoon, and in the evening for the mid north coast, as the low moves south.

Rainfall of 300-700mm had been recorded for northern parts of the Northern Rivers in the last 24 hours, the BOM said.

Dunoon, a small village located roughly 20km north of Lismore, registered a whopping 775mm of rain during the 24 hours ending at 9am on Monday.

Weatherzone, a weather monitoring service, said the "deluge is the second highest daily rainfall total ever officially observed in NSW".

The wider impacted area is vast, with potentially life-threatening flash-flooding taking in swathes of the state.

The BOM said Murwillumbah has seen record flooding with its levee overtopped.

At Coraki, on the flooded Richmond River, 25km south of Lismore, local Shelly Hayes fled her house to stay with a friend, who lives on a hill.

"The main street is all under water. The river is higher than I've ever seen it," Ms Hayes told AAP.

Some of the town's 2000 people were sheltering at the Uniting Church but there was "no bedding or food", she said.

"Looking out my window ... it looks like I live on a dam," newsagency owner Jenelle Stanford who lives between Mullumbimby and Byron Bay told AAP.

One man died when his Land Cruiser was carried away by floodwaters on the Central Coast, north of Sydney, on Friday and another remains missing in floodwaters in Lismore.

Intense episodes of wild weather will remain a worrying reality prompted by climate change, scientists say.

"Over the past decades we have already seen an increase in the number and intensity of extreme rainfall events and we are expecting this trend to continue into the future," said Dr Nina Ridder, a research associate at UNSW Climate Change Research Centre.

Australian Associated Press