Premier Dominic Perrottet apologised to thousands Northern Rivers residents whose homes and businesses were destroyed when towns like Lismore and Ballina were inundated by floodwaters last week.

He admitted the planning and response to the flood disaster was found wanting and said the priority would be tho find housing for those with nowhere to live.

Many people have struggled for days since to access basics such as food, water, power, fuel, phones, ATMs, the internet and medical supplies.

“I’m very sorry … but we’re doing everything that we can,” Mr Perrottet told the Nine Network from Lismore.

“Some of those areas will look back at this and say it could have been done better but … we’re doing absolutely everything we can to provide care and support for those who need it to get those essential supplies in,” he said.

The emergency response would be reviewed, he added.

As complaints from residents grow about a lack of support for communities hit hard by the natural disaster, Perrottet said he would take any financial support from the Commonwealth he could.

Criticism has been levelled at the federal government for not spending money from its $4.8 billion emergency response fund on flood mitigation measures in the wake of the disaster.

“If you’re going to use that fund for a disaster, you’re not going to get a bigger disaster than this one,” Perrottet told ABC TV.

“What has occurred up here is an unprecedented catastrophe.”

Floods have claimed six lives in NSW, including four in Lismore.

And the threat has not receded yet, with the Bureau of Meteorology issuing a severe weather warning for the coast from Coffs Harbour in the north to Bega in the south.

There’s a risk of thunderstorms, large and powerful surf and flooding at levels similar to last week.

The Australia Defence Force is helping with a “mammoth” clean-up with 2000 personnel on the ground across the Northern Rivers.

“We’re getting supplies in. We’re getting food in,” Perrottet said.

Around 2000 of the 3500 homes assessed so far in the Northern Rivers are no longer habitable, increasing the need for short-term and medium-to-long-term accommodation.

Telstra on Monday said it had restored about 80 per cent of mobile coverage for communities in northern NSW and 75 per cent of landline connections.

But some areas remained inaccessible due to continued flooding, road damage or a lack of electricity that is hampering the work of around 1000 technicians.

The northern NSW communities of Kingscliff, Main Arm, Mullumbimby, Ocean Shores, Tucabia, Uki and Wooli are among those still having issues with communications.

The State Emergency Service had another 550 calls for help overnight and performed nine flood rescues.

SES Deputy Commissioner Daniel Austin says volunteers are waiting with “bated breath” to see what Monday Monday.

“We’re certainly on high alert from the Mid-North Coast down into the Shoalhaven,” he told the Nine Network.

“We are watching exceptionally closely a number of catchments … around the Hawkesbury and Central Coast and places.

“The water hasn’t retreated yet. We’re now going to see more flooding on top of what we’ve already seen.”

Six-hourly rainfall totals between 70mm to 120mm are possible on the east coast.

The heavy rain could lead to flash flooding in Greater Sydney, Mid-North Coast, Hunter, Illawarra, South Coast, and parts of the Tablelands.

Multiple flood warnings are already current for parts of NSW, with the Hawkesbury and Nepean regions in danger of more moderate to major flooding.

Thunderstorms could also bring intense rain to the Northern Rivers and damaging winds and large hail could hit Lismore and nearby towns trying to clean up.

Rain is causing significant travel delays with cancellations expected on most Sydney Trains, NSW TrainLink Intercity and Regional trains services.