The unprecedented flood emergency unfolding in northern NSW is likely to worsen in coming days, as thousands of residents across the northern rivers region are ordered to evacuate.
Hundreds of people across the region have been left stranded for hours on rooftops amid the crisis as state and federal emergency services struggled to access impacted areas.
The SES has been overwhelmed with calls for help as the region is hit by its worst rainfall ever, with the situation being described as "dire" and "catastrophic".
There are 12 evacuation orders in place across the state's north, affecting close to 16,000 people, while six areas are under evacuation warnings, covering around 14,000 people.
The entire Lismore CBD is underwater after days of heavy rain that led to the Wilsons River breaching its levee overnight, with roads cut off.
The crisis is widening, with multiple major flood warnings for northeastern NSW including the Tweed, Richmond, Wilsons, Bellinger and Clarence rivers.
State Emergency Services Commissioner Carlene York is co-ordinating a multi-agency response which includes the ADF, NSW Fire and Rescue, police and ambulance, but she admits the weather is making it difficult to get helicopters into the air and boats into the water.
She urged people to heed evacuation warnings, saying otherwise the SES "can't promise to respond at a later stage when you do get into difficulty".
In the past 24 hours there have been 500 flood rescues and 927 requests for help and the premier is warning conditions in the north have not peaked.
Premier Dominic Perrottet says the situation will likely deteriorate and urged those in flood-prone areas to prepare to leave.
"The advice that we have received is that we would expect things to get worse over this period of time," he said on Monday.
"Please do not be complacent.
"Simply because your community has not flooded in the past does not mean that it will not flood over the course of this week. What we are seeing today is unprecedented."
Many residents were surprised by the speed of the inundation overnight, forcing them to take shelter on their roofs.
"It is incredibly distressing, the images that we are seeing," Mr Perrottet said.
The extreme weather is forecast to move further south over the course of the week, with the premier flagging a potential future impact on the south coast, with emergency crews from SA and Victoria being brought in to help with the escalating crisis.
Prime Minister Scott Morrison had offered financial and logistical support to flooded communities, the premier said.
The Bureau of Meteorology expects the far north's heavy rain to ease on Monday afternoon, and in the evening for the mid north coast, as the low moves south.
Lismore recorded 181mm of rain in 30 minutes on Monday and remains the focus of the rescue efforts.
The regional city 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, on Monday evening, prompting fears of unparalleled inundation in the area. The river's previous record high of 12.27 metres occurred in 1954.
The wider impacted area is vast, with potentially life-threatening flash-flooding taking in expansive swathes of the northern rivers and mid north coast, including Coffs Harbour and Bowraville, on Monday.
At Coraki, on the flooded Richmond River, 25km south of Lismore, local Shelly Hayes fled her house to stay with a friend, who lives on a hill.
"The main street is all under water. The river is higher than I've ever seen it," Ms Hayes told AAP.
Some of the town's 2000 people were sheltering at the Uniting Church but there was "no bedding or food", she said.
Meanwhile, 20km 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.
Also on Monday, the SES ordered the town of Mullumbimby to evacuate, 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.
