Parts of the northern rivers have been declared a natural disaster zone after remnants of ex-tropical Cyclone Debbie devastated NSW townships including Murwillumbah and Lismore.

Rainfall of between 500mm and 740mm was recorded in that region in a 24-hour period, inundating homes and businesses with water up to three metres, forcing residents to seek shelter in roof cavities.

Schools were closed, roads transformed into rivers and the damage bill the worst since the $1.2 billion caused by Cyclone Oswald in 2014, but the full extent of the disaster will not be known for some days.

There were concerns late on Friday for the township of Chinderah, with the Tweed River levels expected to rise there further with the high tide. The army was called in to help volunteers from the State Emergency Service to evacuate residents from the town, where the M1 motorway has been blocked by floodwater in both directions.

Some parts of the swollen Richmond River will not peak until Saturday afternoon.

The State Emergency Service carried out 300 flood rescues and had at least another 60 calls that they had not yet been able to reach on Friday evening due to the weather conditions.

"We might get a worst-case scenario there, and find people have deceased overnight as a result of being caught in floodwater," State Emergency Service deputy commissioner Mark Morrow said.

Late Friday, police said a woman's body had been found on a flooded property about 20km south of Murwillumbah.

The woman disappeared in overnight floodwaters from the rural property on Upper Burringbar Road, at Upper Burringbar. A family member found her body on Friday morning.

Another woman's body was found in a car on Friday afternoon, after she was swept away while driving over a causeway in the Hunter Valley on Thursday night.

The 64-year-old woman's was driving on a property in Gungal - around 600 kilometres south of the northern rivers - with her 74-year-old husband when they were washed away around 9pm.

He managed to escape and raise the alarm, however the woman's body was found submerged in four metres of water about 20 hours after she disappeared.

Premier Gladys Berejiklian said 20,000 people had been evacuated from their homes in the state's north and another 10,000 people are without power.

"We're urging everyone up there to apply maximum safety," she said. "Some people won't be returning to their homes for some time."

The Department of Family and Community Services said some of those homes had been lost.

"There's a lot who don't know what they're going to return to - if they can even return to their homes," Leanne Gollan from the Department of Family and Community Services said. "We've had a lot of families up here, a lot of displaced people."

The Tweed River at Murwillumbah peaked at 6.2 metres, slightly higher than the 1954 flood in that town that claimed 15 lives. The Wilsons River at Lismore was expected to peak near 11.57 metres, making it the worst flooding event in there in 15 years, with the town's levee breached at 4am on Friday, triggering an emergency siren through the CBD at that time.

The levee overflow was moving at three metres per second, Lismore mayor Isaac Smith said.

"That will pick up a truck and throw it around," he told AAP.

The mayor said Lismore is a known "flood" town, but many locals haven't faced such a major event in decades.

"We just haven't been in this situation before," he said.

Mr Morrow said it was "far too risky" for emergency services to respond to some people who were trapped, as huge amounts of rain fell in the region in 24 hours.

He says the collision of the ex-tropical cyclone and a cold front was "much more significant" than predicted.

"If you consider 750 millimetres of rain in a 24-hour period, it would've been driving, the wind was up, it was dark, we couldn't get helicopters in the air," he said.

"It happened very quickly. It was a very intensive rain cell over an intermediate period of time. It did not let up. Normally you get a shower, you get a heavy fall. It was continous, and it was far too risky to put emergency services' lives at risk."

Mr Morrow criticised people who have taken to the floodwaters on surfboards and wakeboards and urged people who did not need to go out to avoid doing so for now.

with AAP