About 15,000 people have been evacuated on the north coast and the entire Lismore CBD is inundated after days of unrelenting torrential rain as the town braces for its worst-ever flood crisis that will surpass the devastation caused in 1974 and 1954.
The Bureau of Meteorology says Lismore's Wilsons River could reach around 14.20 metres on Monday afternoon, stoking fears of unprecedented inundation in the area.
The previous high of 12.27 metres was registered in 1954.
Dangerous and rapid river-level rises have been observed along the tributaries upstream of Lismore and heavy rain is falling over the Wilsons River catchment and is forecast to continue during the remainder of Monday morning.
NSW Emergency Services Minister Stephanie Cooke said the situation in Lismore was much more severe than forecast on Sunday, straining the resources of SES.
"We are putting every single available emergency services personnel and resources in place to support the community," Ms Cooke told radio 2GB.
"We appreciate that this situation is absolutely heartbreaking."
The government was "throwing everything" it had at the crisis, and resources from RFS, NSW Fire and Rescue, police and ambulance, and SES were headed to the area as part of a "multi-agency response", she said.
The flood situation in Lismore was "worsening", with hundreds of calls for assistance from the area, and the search for a man missing in floodwaters suspended until conditions allow it to resume, NSW police told AAP on Monday.
Police heard the man call out for help about 4pm on Sunday but lost sight of him a short time later, with reports suggesting he may have been swept into a storm drain.
The BOM has issued a severe weather warning for the Northern Rivers and parts of the mid north coast and Northern Tablelands with the potential for life-threatening flash flooding.
Major flood warnings have been issued for the Tweed River, with major flooding occurring at Murwillumbah, Tumbulgum and Chinderah, while many towns across the north have been evacuated as flooding inundates the region.
Many roads are cut and the State Emergency Service is warning people to heed warnings and avoid entering floodwaters.
Lismore Mayor Steve Krieg described the situation as "very dangerous and life-threatening".
People were caught off guard by the speed of the rising Wilsons River, with some people trapped in their homes and on roofs.
The Richmond River is causing major flooding at Kyogle, Coraki and Bungawalbyn, moderate flooding at Woodburn, and minor flooding at Wiangaree and Casino.
South Murwillumbah has also been evacuated as roads are cut and the only way out is by boat.
The SES ordered the town of Mullumbimby to evacuate on Monday morning, with the area threatened by rapidly rising floodwaters from the Brunswick River.
Fire and Rescue and the SES have been door knocking to ensure people evacuate.
SES volunteers had carried out 70 flood rescues since torrential rain began falling on Tuesday, Premier Dominic Perrottet told reporters on Sunday.
"We know that whilst there might be blue skies in certain parts of NSW that does not mean that there will not be significant flooding events that occur over the course of this week," he said.
One NSW life has already been lost, with a man killed when his LandCruiser was carried away by floodwaters on the Central Coast, north of Sydney, on Friday.