A state of emergency has been declared to tackle the NSW bushfires, with worse weather conditions on the way and forced evacuations likely across many areas of the Blue Mountains, outside Sydney.

Rural Fire Service (RFS) commissioner Shane Fitzsimmons wrote to the premier, Barry O'Farrell, on Sunday requesting he declare a state of emergency.

O'Farrell, who signed the order, said there were concerns weather conditions would deteriorate significantly over the next few days.

"There is potential for a significant and widespread danger to life and property across the state," O'Farrell said in a statement.

"This is not an action taken lightly by either the commissioner, the minister or myself, but it's important the Rural Fire Service and other emergency services have the powers and the resources they need to combat this threat.

"We are planning for the worst but hoping for the best," he said.

The declaration triggers a range of measures, including allowing emergency services to direct the public to evacuate or to order power and gas supplies be shut off.

Earlier Fitzsimmons said the situation in the Blue Mountains was so severe that even built-up areas such as Leura and Katoomba could be in the firing line. He said there were indications the fire at Mount Victoria could threaten homes at Blackheath.

"Conditions are going to be far more concerning over the current days than was otherwise expected," he said. Firefighters were experiencing the worst conditions seen since the 1960s.

"They are in a league of their own," he said. Humidity was set to drop on Sunday night and the bad weather would intensify through Monday, Tuesday and Wednesday.

Residents of the township of Bell were warned to evacuate on Sunday morning as the huge fire between Lithgow and the Blue Mountains was upgraded to emergency status.

Anyone in the area who was unprepared to defend their property should leave immediately and head west to the evacuation centre at Lithgow, the RFS said.

People living in Mount Tomah and Berambing were told to get out urgently shortly before 2pm on Sunday and go to Richmond.

Others, such as those in Bilpin Village, could choose to stay but risked being cut off from power and telecommunication for several days. People in Mount Irvine and Mount Wilson were told it was too late to leave.

O'Farrell grimly warned residents across the state to brace for the possibility of mass evacuations.

"The state's in for challenging days ahead," he told reporters at RFS headquarters on Sunday.

The RFS said 208 homes had been destroyed in the bushfires that have been raging across the state since Thursday.

The premier said hard choices were ahead in the hot, dry conditions.

"That will involve decisions being made about asking people, particularly in fringe areas, to leave their homes for their safety," he said.

"Undoubtedly, that will cause distress to those people and it may cause wider controversy. But it's clear that at times like this it's better to be safe than sorry."

NSW police assistant commissioner Alan Clarke urged people in bushfire-affected areas to heed directions from emergency services.

He said anyone who refused an order to evacuate would be risking their lives, and those of the men and women manning roadblocks and knocking on doors.

"These are extraordinary circumstances," Clarke said. "In the past, we've been able to say 'leave if you can, shelter in place'. Sometimes the message now in these areas will be, 'you must be out'."

But one resident, Craig Jones, said most would probably stay put to defend their homes.

After spending two days helping save his wife's cafe in Bell, Jones said he had cried only once.

That was when he discovered a lone, red waratah in the cafe's garden had survived Thursday's initial intense flames and the subsequent back-burning.

"Mate, I couldn't control myself," he told AAP on Sunday.

Jones and his wife, Eleanor Hajwan-Jones, have been returning to their Bell cafe each day to offer free food and drinks to exhausted firefighters.

"These men and women are fantastic," Jones said. "I can't thank them enough."

Despite the terrifying experience, he said they had no plans to quit.

"This is the Blue Mountains. Tourists will still want to come again here after the fires are gone," he said.

On Sunday evening, fires in Balmoral, Mount Victoria and Springwood remained at a "watch and act" alert as more than 60 blazes burned across the state, with just under 20 out of control.

Interstate fire crews, including eight teams from Queensland and two from the ACT, have been deployed to NSW to help with the effort. Forces in New Zealand are also on stand-by to help when needed.

A total fire ban is in place across Sydney, the Hunter, Shoalhaven, the central ranges and the Illawarra.

Fitzsimmons paid tribute to the firefighters and volunteers who have worked to protect lives and homes from the blazes that broke out on Thursday.

"[We've got] men and women doing some pretty extraordinary work. Time and time again you're hearing extraordinary stories of what they were being confronted with, decisions and actions that they took," he said.

"They've all got each other's back and right at front of mind, they're trying to do all they can for their local communities."

Prime minister Tony Abbott helped out with backburning efforts in the Bilpin area overnight.