Thousands of residents in the northeastern NSW city began evacuating on Monday morning as a number of severe weather and major flood warnings were put in place across the state.

The Bureau of Meteorology is predicting flood levels to peak at 13.5 metres at the Wilsons River on Monday evening, with data showing the highest recorded flood was 12.4 metres in 1880.

BOM warns the river will definitely smash the March 1974 record of 12.15 metres, which is far above the March 2017 peak of 11.59 metres.

The Lismore levee overtopped around 3am and exceeded 10.6 metres.

The slow-moving coastal trough deepened and formed a low-pressure system off the NSW coast on Sunday night, producing even more heavy rainfall, BOM said.

“There were areas of locally intense rainfall associated with embedded thunderstorms,” BOM said.

“Dangerous and rapid river level rises are being observed along the tributaries upstream of Lismore.”

Authorities are saying it is difficult to know just how much water is coming down the valley into the city.

The city centre has been completely closed off to drivers and pedestrians, with police shutting it off as water levels continue to rise in the CBD.

A desperate search was sparked late on Sunday for a man missing in floodwaters at Lismore but it was suspended due to the dangerous waters.

Officers from Richmond Police District were near Brewster Street and Uralba Street in Lismore about 4pm Sunday when they heard a man call for help.

They lost sight of the man and believe he may have gone down a drain, searching for him into the evening.

NSW Police were due to resume the search at first light on Monday.

NSW SES Chief Superintendent Steve Patterson said more rain was predicted for areas across the Northern Rivers and mid-north coast.

“The predictions are that we will see about 150-250mm of rain within 24 hours across that widespread area,” he said.

A number of evacuation orders are in place across NSW on Monday morning, including parts of Lismore, South Murwillumbah, Wiangaree, Tumbulgum, Condong, Mullumbimby, Coraki and Kyogle.

Major Flood Warnings are also in place for the Tweed River, Clarence River, Brunswick River and Marshalls Creek.

BOM is predicting major flooding for Murwillumbah and Tumbulgum, while the Clarence River at Grafton, Ulmarra and Maclean could reach levels similar to those seen in March last year.

Meanwhile, Casino has been warned to prepare for the possibility of flooding and possibly evacuation.

Conditions are worsening across the border in Queensland’s southeast, with Premier Annastacia Palaszczuk describing the carnage as “unpredictable as a cyclone”.

- by Lauren Ferri, news.com.au