Hundreds of people remain unaccounted for and a woman’s body has been found in a flooded home amid devastating scenes in NSW.

NSW Police confirmed the body of the woman, in her 80s, was found in a home in Lismore on Tuesday morning. She is yet to be formally identified.

Police fear many people are still isolated in homes across the northern rivers, as rescue operations continued on Tuesday. Up to 3000 people were rescued on Monday.

Police have urged locals to use the Red Cross’s Register, Find, Reunite website to register their movements.

“As we continue our efforts to rescue as many people as we can from homes in the Northern Rivers, NSW Police also need to account for the whereabouts of a number of people who were rescued yesterday,” State Emergency Operations Controller, Deputy Commissioner Mal Lanyon said.

“We understand communications in the region have been significantly impacted by this weather event, and this can add to the stress of not being able to get in contact with family and friends.”

Also on Tuesday, dozens of people were rescued after spending the night on a bridge outside Lismore as floodwaters rose around them – while warnings for more heavy rain and dangerous weather spread as far south as the NSW south coast.

NSW Emergency Services Minister Steph Cooke said people should expect more fatalities.

“I think that it is unrealistic that a disaster of this magnitude will mean that there are no lives lost,” she said.

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About 50 people and five horses spent Monday night on Woodburn Bridge before their plight was revealed by news helicopters early on Tuesday.

The Richmond River at Woodburn was at 6.3 metres and rising when they were rescued.

“We had no capabilities to get them off in the dark so we just had to make sure that they bunkered down and we went in this morning and got them all out,” Woodburn State Emergency Services unit commander Ashley Slapp said.

Elsewhere helicopters and boats were on the way to rescue about 50 people from a “life-threatening” situation at Bungawalbin on Tuesdsay afternoon.

Thousands of residents have been displaced by the devastating historic flooding in northern NSW and south-east Queensland. On Tuesday, NSW residents were warned to brace for more thunderstorms, heavy rain and damaging winds as the extreme weather moved south towards Sydney.

A severe weather warning has been issued for heavy rain and high winds along the NSW coast from Newcastle to Bega, which could cause widespread damage and flooding.

The wild weather is expected to hit on Tuesday night – with up to 200 millimetres of rain forecast to fall in six hours in some areas.

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Meteorologist Dean Narramore said high winds and hazardous surf conditions were also forecast, with flood warnings for several regions.

Authorities are monitoring the low-pressure system off the NSW coast, and expect it to affect several areas in Sydney’s west this week.

“We’re very interested in the Hawkesbury and Nepean area,” Mr Narramore said.

“We’re expecting moderate flooding, and a watch for moderate to possibly major flooding, but definitely moderate at this stage.”

The State Emergency Service has put out a flood watch for the whole Sydney region, with the worst predicted for areas around the Upper Nepean River. Ms Cooke urged residents to prepare.

“People in Sydney and across the south coast have time. The time to prepare is now,”  she said.

NSW Premier Dominic Perrottet urged people returning to work in Sydney’s CBD to be careful.

“It’s going to be a difficult week in our city, as we’ve seen those heavy rains go from the north. We’re seeing it in Sydney today and we’d expect that to go further south towards the back end of the week,” he said.

“We say to people across our city and across our state, look after yourself, follow the advice, stay safe and we’ll all get through.”

Mr Perrottet said the clean-up “and getting communities back on their feet will take weeks, months and years”.

“There’s still difficult times to go,” he said.

An Army helicopter flies over flood-affected properties in near Ballina, in northern NSW. Photo: ADF

Back in Lismore, Mr Slapp said the flood situation was “devastating”. The Wilsons River peaked at a record of 14.4 metres in the town on Monday night.

“The capabilities of rescues have increased dramatically today, we have helicopters picking people up off roofs,” he said.

Rescuers in a flotilla of dinghies and inflatables have helped plucked stranded Lismore residents from rooftops and balconies of submerged homes.

SES spokesman Josh Clarke said helicopters and boats were being sent to about 50 people stranded at Bungawalbin, but getting them to safety could take some time.

“Crews will be there but we have to actually get down there and check that there’s no one in the roof cavity, and in some cases cut into the roof to make those rescues,” he told the ABC.

Towns across northern NSW, including Lismore and Ballina, are underwater and rivers are peaking as the rain finally stops.

The unprecedented floods are much worse than anticipated, with 35,000 people ordered to evacuate their homes and another 310,000 warned to be ready to flee.

People in South Ballina were told it was too late to leave on Tuesday morning as floodwaters hit the northern coastal town.

The SES carried out 932 flood rescues across the northern rivers region in the 24 hours to Tuesday morning.

Five helicopters were helping perform rescues in the Ballina region, along with 46 water rescue personnel and 500 volunteers.

The crisis has engulfed NSW’s north-east, with multiple major flood warnings including the Tweed, Richmond, Wilsons, Brunswick, Bellinger and Clarence rivers.

Disaster assistance is available in 17 local government areas. They are: Armidale, Ballina, Bellingen, Byron, Clarence Valley, Coffs Harbour, Glen Innes Severn, Hornsby, Kempsey, Kyogle, Lismore, Nambucca, Port Macquarie/Hastings, Richmond, Tenterfield, The Hills and Tweed.

-with AAP