Emergency Services Minister David Elliott warned residents were facing what "could be the most dangerous bushfire week this nation has ever seen".

The NSW fires have claimed three lives and so far destroyed more than 150 homes.

Premier Gladys Berejiklian said the last time a state of emergency was declared in the state was 2013 when there were extensive bushfires in the Blue Mountains.

Ms Berejiklian warned people to "for heaven's sake, stay away from bushland" on Tuesday.

"The catastrophic weather conditions mean that things can change very quickly," she told reporters on Monday.

"You might think you're OK and a few minutes later you won't be. Please heed all the messages you receive. Tomorrow (Tuesday) is not the day to be complacent."

Mr Elliott said the state of emergency was precautionary but necessary.

"We have tools like state of emergency available to us to ensure there is no legal barrier, there are no operational barriers, to ensure that the people of the Rural Fire Service (can) do what they're meant to do," the minister said.

There are currently 60 fires burning across NSW with more than half uncontained.

"Catastrophic is off the conventional scale," RFS Commissioner Shane Fitzsimmons said.

"We are talking about indices that go well beyond the old scale of 100."

The blazes are raging from the northern border with Queensland down to the mid-north coast, out to the state's central west and south toward the Illawarra.

Catastrophic fire danger has been declared for the Sydney and Hunter regions on Tuesday with severe and extreme danger across vast tracts of the rest of the state.

Australian Associated Press