Eleven flood warnings are current in northern NSW as the big wet which has inundated Queensland's southeast and claimed seven lives heads down the east coast.
Dominic Perrottet said an evacuation alert is also in place for residents along the Clarence River near Maclean.
SES volunteers had already affected 70 flood rescues since torrential rain began falling on Tuesday including five in the past 24 hours, the premier told reporters on Sunday morning.
"We know that whilst there might be blue skies in certain parts of NSW that does not mean that there will not be significant flooding events that occur over the course of this week," he added.
"So please, please, do not be complacent."
Severe weather is impacting a 450 kilometre stretch from the Queensland border to Port Macquarie with a risk of life-threatening flash flooding particularly during Sunday afternoon and evening, says the Bureau of Meteorology.
The Northern Rivers and Mid North Coast regions are expected to see "widespread, high rainfall totals" exceeding 150mm.
NSW Emergency Services Minister Steph Cooke said there are 550 SES volunteers in field and strong back-up plans in place.
"The SES is well supported by our other emergency services, organisations, including the RFS, where we have pre-positioned aircraft up on the far north coast," she said.
"They're able to respond to any incidents that arise up there. We also have fire and rescue, with both teams and appliances in the north of the state, again ready to respond the risk of flash flooding, heavy rainfall."
Senior Bureau of Meteorology hydrologist Ailsa Schofield said there is the potential, as the monster system moves south, for up to 250mm of rain in some areas.
"We won't start to see the rainfall ease until tomorrow," she said.
"What this means for communities is that there is very serious and life-threatening flash flooding and riverine flooding likely.
"We have a number of flood warnings current from minor, moderate and up to major flooding."
There five catchments affected are the Tweed, Wilsons, Richmond, Clarence and Arara rivers.
The communities of Tombolgin, Lismore, Grafton, Coots Crossing, Kyogle and Coraki have major flood warnings forecast from Sunday or for Monday.
Ms Schofield said it was possible the system could reach southern NSW by Wednesday.
One NSW life has already been claimed, with a man killed when his LandCruiser was carried away by local floodwaters on the Central Coast, north of Sydney, on Friday.
Meanwhile, communities affected by severe storms and floods across NSW in February and March last year will receive an additional $200 million in a jointly funded Commonwealth-state support package.
Federal Emergency Management Minister Brigit McKenzie says the money will go to infrastructure recovery programs and community mental health initiatives.
