The flood 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 is warning NSW is in for more thunderstorms, heavy rain and damaging winds as extreme weather moves south along the coast.
It says 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 9.70 metres until midday on Tuesday.
Hundreds of people are still unaccounted for and at least one man is feared dead after a man went missing in flood waters.
NSW Emergency Services Minister Steph Cooke says 350,000 people in the northern rivers and north coast areas of the state are on high alert under evacuation orders or warnings.
There are 26 evacuation orders in place covering around 40,000 people, with those in west Ballina the latest to be ordered to flee their homes.
There are five evacuation warnings in place covering 308,000 people.
"So if you put that all together we are talking about 350,00 people in that area who are currently on high alert and impacted by this unfolding, unprecedented event," she said.
"Please prepare to leave at very short notice," she told Sydney radio 2GB.
The situation could escalate very quickly, she said.
"Please have your bag packed ... be ready to go."
Emergency services were overwhelmed with calls for help on Monday and 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 response crews were overwhelmed by the scale of the crisis, 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 homes was "just devastating ... the entire building was underwater".
"It was quite terrifying trying to get those elderly people out of their homes," he told Sydney radio 2GB.
Shelly Hayes, from the flooded area of Coraki about 25 km 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.
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