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People in South Ballina have been told it is too late to leave as floodwaters hit the northern NSW town.

The State Emergency Service performed 932 flood rescues across the Northern Rivers region - which encompasses Ballina - in the 24 hours to Tuesday morning.

There are evacuation orders covering 34,000 people and another 310,000 across the state are on standby.

People in the other parts of Ballina have been ordered to evacuate as unprecedented flooding continues to devastate vast swathes of the NSW north coast.

"If you are under an evacuation order please evacuate immediately unless it is not safe to do so," an SES spokesman said.

"Do not go through dangerous floodwater to get out. Move to higher ground where possible."

The crisis has engulfed the northeastern part of the state, with multiple major flood warnings including the Tweed, Richmond, Wilsons, Brunswick, Bellinger and Clarence rivers.

The Bureau of Meteorology warned NSW is in for more thunderstorms, heavy rain and damaging winds as extreme weather moves south along the coast.

A developing low pressure system is heading towards Sydney on Tuesday night and Wednesday morning, with heavy rain that could cause flash flooding and potentially hazardous conditions.

Thousands of people have been made homeless by flooding on the north coast with many spending the night in evacuation centres.

The unfolding crisis is at its worst in the northern rivers city of Lismore, which remains submerged after the Wilsons River peaked at 14.4 metres at 3pm on Monday before starting to fall.

River levels are expected to remain above the major flood level of 9.7 metres until midday on Tuesday.

Hundreds of people are still unaccounted for and at least one man is feared dead after he went missing in flood waters.

NSW Emergency Services Minister Steph Cooke said 350,000 people in the northern rivers and north coast areas are on high alert under evacuation orders or warnings.

"Please prepare to leave at very short notice," she told Sydney radio 2GB.

"Please have your bag packed ... be ready to go."

Emergency services were overwhelmed with calls for help on Monday. Hundreds of people across the region were stranded for hours on rooftops as state and federal emergency services struggled to access impacted areas.

Premier Dominic Perrottet described the flooding and its impacts as "unprecedented" and "distressing".

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

Disaster assistance is now available in 17 local government areas in response to severe storms and flooding, which began on February 22 and continues to impact large areas of northern NSW.

The LGAs 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.

Help is being provided through the jointly funded Commonwealth-State Disaster Recovery Funding Arrangements.

As emergency crews were overwhelmed, people pitched in to help each other.

About 45 recently arrived Fijian abattoir workers helped rescue about 60 residents from a Lismore nursing home that was inundated.

Apenisa Marau said the scene at the home was "just devastating ... the entire building was underwater".

"It was quite terrifying trying to get those elderly people out of their homes," he told 2GB.

Shelly Hayes, from the flooded area of Coraki about 25km south of Lismore, was worried about food supplies

"As a town we're going to need a food drop because all the roads are cut off, so we can't get out and nothing can come in," she told AAP.

© AAP 2022

The unprecedented floods in northern NSW are worse than anticipated, with 34,000 people ordered to evacuate and another 310,000 warned to be ready to flee.

State Emergency Service Commissioner Carlene York said workers and volunteers were "flat out" amid distressing scenes as the waters leave a trail of destruction.

"We've been planning for this but yes, (the rain) was a lot heavier and it's staying around longer than we had anticipated," Ms York told Sydney radio 2GB on Tuesday.

"Worse than we had anticipated but we've managed to put a lot of resources down there."

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

The SES performed 932 flood rescues across the Northern Rivers region - which encompasses Ballina - in the 24 hours to Tuesday morning.

People in other parts of Ballina have been ordered to evacuate as unprecedented flooding continues to devastate vast swathes of the state's north coast.

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

The SES was getting "distressed" calls from people who couldn't find loved ones, Ms York said.

She urged people to register the missing with the Red Cross and said seeking shelter with family and friends would ease the pressure on stretched evacuation centres, which are housing 1000 flood victims.

Ballina Mayor Sharon Cadwallader said the town is doing better than some of its neighbours, with floodwaters moving slowly.

"This is a one-in-500 year flood we are experiencing. It is an unprecedented event and it is a most serious situation," Cr Cadwallader told the Seven Network.

The crisis has engulfed the northeastern part of the state, with multiple major flood warnings including the Tweed, Richmond, Wilsons, Brunswick, Bellinger and Clarence rivers.

The Bureau of Meteorology warned NSW is in for more thunderstorms, heavy rain and damaging winds as extreme weather moves south along the coast.

A low pressure system is heading towards Sydney on Tuesday night with heavy rain that could cause flash flooding and potentially hazardous conditions.

Thousands of people have been made homeless on the north coast with many spending the night in evacuation centres.

Lismore remains submerged after the Wilsons River peaked at 14.4 metres at 3pm on Monday before starting to fall.

Hundreds of people are still unaccounted for and at least one man is feared dead after he went missing in floodwaters.

Emergency Services Minister Steph Cooke said 350,000 people in the northern rivers and north coast areas are on high alert under evacuation orders or warnings.

"Please prepare to leave at very short notice," she told 2GB.

Emergency services were overwhelmed with calls for help on Monday with hundreds of people stranded for hours on rooftops as state and federal emergency services struggled to get to them.

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

Disaster assistance is now available in 17 local government areas.

The LGAs 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.

As emergency crews were overwhelmed, people pitched in to help each other.

About 45 recently arrived Fijian abattoir workers helped rescue about 60 residents from a Lismore nursing home that was inundated.

Apenisa Marau said the scene at the home was "just devastating ... the entire building was underwater".

"It was quite terrifying trying to get those elderly people out of their homes," he told 2GB.

© AAP 2022

The unprecedented flood emergency unfolding in northern NSW is likely to worsen in coming days, as Lismore is expected to be submerged under 16 metres of flood water.

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.

© AAP 2022