​The NSW super storm will be declared a natural disaster by the state government, which has promised financial assistance to the worst-hit areas now contemplating a massive clean-up.

Premier Mike Baird said on Wednesday morning that a natural disaster declaration would be finalised within the next 24 hours, "and obviously areas such as Maitland and Dungog and parts of the Central Coast will be covered by this".

"Every community can have this assurance: that we will do everything possible we can to help them get back on their feet," Mr Baird said.

In Dungog, a Hunter Valley town where flood waters swept away homes and claimed three lives, more rain had fallen in the previous 24 hours than in any 24-hour period "for the past century," he said.

"It is a huge unprecedented amount of water, which is why we have seen some of the impacts we have," Mr Baird said.

"Obviously the next stage that we want to focus on is the recovery but we do need to get through these 24 hours so the focus is to get through the weather and then the recovery effort will get underway."

The declaration would enable local governments, affected businesses and those suffering hardship to access financial assistance from the state government to aid the recovery process.

"There is a range of formulas available but in simple terms there will be grants that are available to councils, to businesses, to families in individual circumstances that will be worked through as part of that process," Mr Baird said.

But Mr Baird said restoring power to more than 200,000 homes was the immediate priority.

Vince Graham, the chief executive of Networks NSW asked the community to be patient after 130,000 homes in the Newcastle, Hunter and lower north coast region lost power, in addition to a further 80,000 on the Central Coast and 15,000 in Sydney.

"The sheer magnitude of this storm event will mean that there will be some extended days before we are able to restore power and I expect that it will be well into the weekend before we are mopping up the tail of this activity," Mr Graham said on Wednesday.

"Given the way this storm front is moving to the Illawarra," Mr Graham said, "it may very well be that the Illawarra is added to the list of geographic areas where we have substantial supply problems."

Despite the expectation that conditions would ease heading into Thursday, Mr Baird warned about the ongoing risk of flash flooding "and indeed even when the sun comes out there's still a lot of movement in the rivers".

"The weather still remains dangerous and the conditions are dangerous so this is not a time for complacency," he said.

There had been about 100 rescues after people "either by accident or by act," became trapped after entering floodwaters, State Emergency Service commissioner Adam Dent said.

"I cannot stress enough, please stay out of floodwaters. It is just not worth it," Mr Dent said.

NSW Attorney-General Gabrielle Upton reminded those affected by the storms that they could access free legal help if they had questions about their insurance cover and claims.

"Getting legal advice early is critical for people whose home and property has been damaged by heavy rain and storm," Ms Upton said.

"Anyone can get free legal advice by calling 1800 801 529 or contacting their local Legal Aid office."