About 15,000 people have been evacuated across the north coast and the entire Lismore CBD is inundated after days of heavy rain that led to the Wilson River breaching its levee overnight.
Many of those who clambered on roofs in the middle of the night have spent hours waiting to be rescued after being caught unawares by the speed of the rapidly rising waters.
The ADF arrived in Lismore on Monday to help the stretched emergency response crews which includes the RFS, NSW Fire and Rescue, police and ambulance, and State Emergency Services volunteers.
Prime Minister Scott Morrison told reporters a "crisis situation" was under way in northern NSW, and federal authorities were working closely with the state government to give "every support" to those impacted by the rising waters in Lismore.
The ADF efforts in Lismore had been hampered by the extreme weather causing problems accessing to 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 national security committee of cabinet on Tuesday.
Lismore recorded 181mm of rain in 30 minutes on Monday and it remains the focus of the emergency, facing its worst-ever flood crisis that is set to surpass the devastation caused in 1954 and 1974.
The town's Wilsons River is expected to reach around 14.2 metres on Monday afternoon, 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 the 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 that's forecast to continue during the remainder of Monday.
Lismore Mayor Steve Krieg described the situation as "unprecedented", "very dangerous and life-threatening", while federal Labor MP for Lismore Janelle Saffin said she escaped rising waters by swimming out of a house where she was sheltering after sudden flooding left her a "sitting duck".
"It came down to 'We're going to swim' and I don't advise everyone to do that but there was no choice," Ms Saffin told Sydney's 2GB radio.
"I'm worried about people on roofs and all hands are on deck, all our emergency services, everybody."
NSW Emergency Services Minister Stephanie Cooke said Lismore's situation was much more severe than forecast on Sunday, straining the resources of SES.
The government was "throwing everything" it had at the crisis with a "multi-agency response", she said.
Twenty kilometres north of Lismore, at Rocky Creek Dam, the SES is directing people below the dam to evacuate to higher ground as as "uncontrolled water is spilling over", amid fears it could burst its banks.
While the Lismore area is the crisis epicentre, the emergency is spreading with the Bureau of Meteorology issuing multiple major flood warnings that remain current 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 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 also 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.
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.
