Bushfires raged out of control across Australia's most populous state on Tuesday, fanned by intense heat and high winds in "catastrophic" conditions that threatened homes and triggered evacuations.

More than 130 fires were burning across New South Wales state, 40 of them uncontained, state Rural Fire Service commissioner Shane Fitzsimmons told reporters in Sydney.

"You don't get conditions worse than this. We are at the catastrophic level," he said.

Introduced after the 2009 Black Saturday firestorm in Victoria state, which claimed 173 lives, a "catastrophic" rating means fires will be uncontrollable, unpredictable and fast-moving, with evacuation the only safe option.

New South Wales faced one of the highest-risk fire days in its history, fanned by high winds and temperatures reaching 45 C in parts of the state.

"Fortunately, we've got no reports of people's homes being damaged or destroyed at this stage," Fitzsimmons said.

While no deaths had been reported, officials remained on alert.

Shoalhaven mayor Joanna Gash said the area was a "tinderbox." Gash said the extreme conditions had not been seen before. "A catastrophic fire is one you don't really fight. You just try to get people to safety," she said.

Much of southern Australia is enduring a summer heatwave and a total fire ban is in place throughout New South Wales, while all national parks are closed, with temperatures topping 42 C in Sydney and hitting 45 in the state's west.

AFP