The flood emergency in northern NSW is widening with three rivers now at major risk of breaking their banks, as the city of Lismore faces an "unprecedented" crisis as thousands evacuate amid rapidly rising waters.
About 15,000 people have been evacuated across the north coast and the entire Lismore CBD is inundated after days of unrelenting torrential rain that led to the Wilson River breaching its levee overnight.
People were caught unawares by the speed of the rapidly rising waters, with some forced to take refuge on the roofs of their inundated homes overnight.
The ADF arrived in Lismore on Monday morning to help the stretched emergency response crews which includes the RFS, NSW Fire and Rescue, police and ambulance, and State Emergency Services volunteers.
The town recorded 181mm of rain in 30 minutes on Monday and a severe thunderstorm warning is current for intense rainfall across the northern rivers.
The Bureau of Meteorology issued a major flood warning at 9.42am on Monday for the Tweed River, adding to major warnings already in place for the Wilson and Clarence rivers as large swathes of the northern rivers grappled with the crisis.
Near the Tweed River, the BOM said major flooding was occurring at Tumbulgum, Mueillumbah and Chinderah, with further rain forecast for Monday, stoking fears waters could rise even higher.
"This has caused major flooding at Tumbulgum and Murwillumbah, with further rises possible during Monday. At Chinderah, major flooding is occurring on the high tide," it said.
Lismore remains the focus of the emergency, with reports of people are climbing onto their roofs, and an elderly couple is stuck in a roof cavity as water rises around them.
The town is bracing for its worst-ever flood crisis that will surpass the devastation caused in 1954 and 1974, with Wilsons River set to reach around 14.20 metres on Monday afternoon, prompting fears of unparalleled inundation in the area.
The river's previous record high of 12.27 metres was registered in 1954.
Dangerous and rapid river-level rises have been observed along the tributaries upstream of Lismore, with heavy rain falling over the Wilsons River catchment and forecast to continue during the remainder of Monday morning.
The floods at Lismore were "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.
Lismore Mayor Steve Krieg described the situation as "unprecedented", "very dangerous and life-threatening".
"This really is a catastrophic event for the city of Lismore. We have broken records, flood records dating back to 1854 already ... The rain has set in and there is another forecast of another 200mm of rain today," Mr Krieg told the Nine Network.
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.
"If you remain in the area you will be trapped without power, water and other essential services and it may be too dangerous to rescue you," the SES said.
NSW Emergency Services Minister Stephanie Cooke said Lismore's situation 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 Sydney radio 2GB.
The government was "throwing everything" it had at the crisis with a "multi-agency response", she said.
Many roads are cut and the SES is warning people to heed warnings and avoid entering floodwaters.
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.
One man died when his LandCruiser was carried away by floodwaters on the Central Coast, north of Sydney, on Friday and another remains missing in floodwaters in Lismore.
© AAP 2022