About 15,000 people have been evacuated from their homes across the north coast and the entire Lismore CBD is under water after days of heavy rain that led to the Wilsons River breaching its levee overnight.
Many residents were surprised by the speed of the inundation, forcing them to take shelter on their roofs in the early hours of Monday morning.
The ADF arrived in Lismore on Monday to help stretched rescue and emergency response crews from the RFS, NSW Fire and Rescue, police and ambulance, and State Emergency Services volunteers.
Despite the efforts, Lismore Mayor Steve Krieg said hundreds were still trapped amid the flooding in what he called a "dire situation" for the regional city.
"We're doing our absolute best to co-ordinate everything as best we can with the resources that we have to get everyone out of the low-lying areas," Mr Krieg told ABC TV on Monday afternoon.
It is something that no-one in this area has ever seen before."
Earlier, Prime Minister Scott Morrison told reporters federal rescue efforts in Lismore had been hampered by the extreme weather, which was causing problems accessing the town from the air.
"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."
He planned to meet on the "very distressing situation" with the government's emergency management authority in Canberra on Monday afternoon, before further discussions with the national security committee of cabinet on Tuesday.
NSW Premier Dominic Perrottet will address media on the crisis later in the day.
Lismore recorded 181mm of rain in 30 minutes on Monday and remains the focus of the rescue efforts.
The town is facing its worst-ever flood crisis, surpassing the devastation caused in 1954 and 1974.
Expectations for the Wilsons River have been upgraded to a forecast peak of 14.40 metres, prompting fears of unparalleled inundation in the area. The river's previous record high of 12.27 metres occurred in 1954.
Police told AAP they fielded hundreds of calls for help from the area and a search for a man missing in floodwaters has been suspended until conditions allow it to resume.
Dangerous and rapid river-level rises have been observed along the tributaries upstream of Lismore, with heavy rain falling over the Wilsons River catchment forecast to continue during the remainder of Monday.
Emergency Services Minister Stephanie Cooke said Lismore's situation was much more severe than forecast on Sunday, straining SES resources.
Twenty kilometres north of Lismore, at Rocky Creek Dam, the SES is directing people below the dam to evacuate to higher ground as "uncontrolled water is spilling over", amid fears it could burst its banks.
While the Lismore area is the epicentre of the disaster, the emergency is widening, with the Bureau of Meteorology issuing multiple major flood warnings for northeastern NSW including the Tweed, Richmond, Wilsons, Clarence and Brunswick rivers, and Marshall Creek.
The area impacted is vast, with flash-flooding set to continue for parts of the northern rivers and mid north coast, including Coffs Harbour and Bowraville, on Monday.
"Rain will continue today and gradually ease in the far north later this afternoon and later this evening for parts of the mid north coast as the low moves a little further south," the BOM said.
The SES ordered the town of Mullumbimby to evacuate on Monday morning, with the area threatened by rapidly rising floodwaters from the Brunswick River.
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.
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