The Bureau of Meteorology says rainfall of between 300 millimetres and 700mm has been recorded for the northern parts of the NSW Northern Rivers in the past 24 hours.

The high rainfall has seen the Lismore levee overrun on early Monday morning and record flooding above the March 1976 peak and February 1954 levels is occurring in the town.

The highest flood on record peaked at 12.46 metres in 1890.

A Lismore man has spoken of his terrifying ordeal waiting on a rooftop to be rescued.

Nat Hall, 29, who has physical and intellectual disabilities, was perched on the roof of his apartment for about 90 minutes before being rescued by members of the community.

His housemate Mel, as well as the occupants of three other units, were also rescued.

"I was absolutely terrified," he said.

"We were on the roof. There was no other way. The doorway to get out of the apartment was under water.

"Watching the water pull poles out of the ground, pull trees out of the ground; it was absolutely terrifying."

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Prime Minister Scott Morrison said it was a "crisis situation" in northern NSW, and federal authorities were working closely with the state government to give "every support" to those impacted by the rising waters in Lismore.

The extreme weather was impeding the Australian Defence Force efforts to access the town.

"We are somewhat encumbered by the weather system which does frustrate aerial operations for evacuation," Mr Morrison said.

"There are reports of many people on roofs and emergency services who are getting people on roofs, then themselves having to be air evacuated."

Lismore MP Janelle Saffin said she escaped rising waters by swimming out of a house where she was sheltering after sudden flooding left her a "sitting duck".

"It came down to 'We're going to swim' and I don't advise everyone to do that but there was no choice," Ms Saffin told Sydney's 2GB radio. "I'm worried about people on roofs and all hands are on deck, all our emergency services, everybody."

The emergency continues to spread with the Bureau of Meteorology issuing multiple major flood warnings that remain current for northeastern NSW including the Tweed, Richmond, Wilsons, Clarence and Brunswick rivers, and Marshall Creek.

Premier Annastacia Palaszczuk says life-threatening flash-flooding is under way and some rivers are still yet to peak, particularly in Logan and Gold Coast.

Major flooding continues in the Gympie, Maryborough and Sunshine Coast regions north of Brisbane after widespread rainfall saturated the area with totals of 200-600 millimetres recorded in the past four days.

More than 3500 homes in Gympie have been affected after more than 1000mm of rain was dumped on the region in that time.

Flood warnings will remain in place in Gympie, Maryborough and Noosa as more flooding is likely on Monday evening.