Australian fire crews were bracing Sunday for some of the worst conditions in decades as several major blazes threatened to merge into a mammoth firefront that could reach Sydney.

More than 200 homes have already been destroyed and another 120 damaged by the wildfires, which broke out across New South Wales state in unseasonably warm weather last week, fanned by extremely high winds.

The worst of the blazes, in the Blue Mountains west of Sydney, plunged the city last week into an eerie midday darkness as plumes of smoke and ash filled the sky.

One man has died so far trying to protect his property.

Three separate infernos continued to burn in the Blue Mountains on Sunday. Grim forecasts of intensifying heat and winds prompted NSW Premier Barry O'Farrell to declare a state of emergency giving firefighters the power to forcibly evacuate people, with penalties for refusing.

"This is not an action taken lightly... but it's important the Rural Fire Service (RFS) and other emergency services have the powers and the resources they need to combat this threat," said O'Farrell.

"We are planning for the worst but hoping for the best."

Officials issued dire predictions about the worsening weather forecasts through to Wednesday, with a RFS spokesman saying there was the "very real potential that these three fires -- (one) in Lithgow and the two in the (Blue) Mountains -- could form as one fire over the next couple of days".

"We can understand the magnitude of that as it would then creep into the bottom end of Sydney. It's certainly something that we're very concerned about," the spokesman said.

Sydney's suburban outskirts are just across the Nepean River from the foot of the mountains. Embers jumped its banks on Thursday, starting a fire at Castlereagh near Penrith.

Some 76,000 people live in the Blue Mountains while the population of Sydney is 4.4 million.

NSW RFS Commissioner Shane Fitzsimmons said there would be several extremely difficult days ahead for fire crews, with temperatures set to soar and a return to dangerously high winds.

"We've got what would be unparalleled (conditions) in terms of risk and exposure for the Blue Mountains and Hawkesbury communities throughout this week," Fitzsimmons told reporters.

"If you are to draw a parallel, and it's always dangerous to draw a parallel, at best you'd be going back to time periods in the late 60s."

"The reality is, however, these conditions that we're looking at are a whole new ball-game and in a league of their own."

A total ban on starting open-air fires for barbecues or other reasons was in place in Greater Sydney and three other regions across the state until further notice.

An emergency warning was issued for the Blue Mountains village of Bell, where residents were urged to evacuate due to the immediate threat of fire. Other township residents were told to shelter in their homes or warned that they faced several days of isolation without electricity.

State Assistant police commissioner Alan Clarke said mandatory evacuation orders would be enforced in some areas, describing the risk as "far more extreme" than in past fires.

"Police will be doing forced evacuations if the risk is necessary," Clarke told reporters.

"At the end of the day we hope we have buildings standing, but if we don't have buildings standing we don't want bodies in them."

Wildfires are common in Australia's summer months, which run from December-February. But an unusually dry and warm winter and record spring temperatures has seen the 2013/14 fire season start early.