The NSW Premier has declared a seven-day state of emergency, and the fire chief is predicting "the most dangerous bushfire week this nation has ever seen".

Key points: NSW frontline agency commissioners made a "unanimous" decision to declare a state of emergency

NSW frontline agency commissioners made a "unanimous" decision to declare a state of emergency The last state of emergency was declared during the 2013 Blue Mountains fires, the Premier said

The last state of emergency was declared during the 2013 Blue Mountains fires, the Premier said There are "catastrophic conditions" of high temperatures and winds in the Greater Sydney and Hunter regions on Tuesday

Last night, Rural Fire Service (RFS) Commissioner Shane Fitzsimmons wrote to Gladys Berejiklian and asked her to grant him emergency powers under the State of Emergency and Rescue Management Act.

Authorities announced this afternoon they would also be extending the catastrophic fire dangers to Illawarra and Shoalhaven in addition to the Greater Sydney and Greater Hunter regions.

The NSW Education Department also announced over 350 schools would be closed on Tuesday.

"Our state has already been hit by some of the most devastating bushfires we have ever seen, with three lives lost and more than 150 structures destroyed," Ms Berejiklian said.

"With catastrophic weather conditions predicted for this week, particularly Tuesday with hot weather and strong winds, I have decided to take the Commissioner's advice and make this declaration.

"No matter where you are … everybody has to assume the worst and we cannot allow complacency to creep in."

Yesterday a blaze tore through the dense trees of Johns River near Port Macquarie. ( ABC News: Mridula Amin )

NSW Police Minister David Elliot said the decision to declare a state of emergency was a "unanimous" decision by all of the commissioners of the state's combat agencies.

He said the decision would allow the RFS to "do what they're meant to do" and not be impeded by any "legal barrier".

"This is a tool that government very, very rarely use but it is a tool that is necessary," Mr Elliot said.

"[This] potentially could be the most dangerous bushfire week this nation has ever seen."

The declaration grants emergency powers to the RFS including the coordination of evacuations, extended access to government resources, traffic redirections, the power to shore up or demolish buildings, and power to shut down gas, electricity, oil and water services.

Disobeying orders under these powers, including orders to the public to leave an area, would be illegal.

The last state of emergency in NSW was issued in October 2013 by then-premier Barry O'Farrell during bushfires in the Blue Mountains.

The Premier said the declaration was a "precautionary stance" and advised NSW residents to be on high alert.

Over 150 houses have been destroyed so far as bushfires have ravaged land, property and animal habitat. ( ABC News: Mridula Amin )

"If you're told to evacuate, please do so. If you're told to take certain actions, please do so," she said

"And for heaven's sake, stay away from bushland tomorrow."

The RFS said more than half of the 60 fires burning across the state remained uncontained.

"The fire behaviour is still quite volatile and there are still lots of communities that remain at risk today, Commissioner Fitzsimmons said.

"Not only are we dealing with an extraordinary amount of fires in north-eastern New South Wales and elsewhere, tomorrow we are facing some horrendous conditions."