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 degrees Celsius (113 degrees Fahrenheit) 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, with particular concerns about the regions of Shoalhaven, Illawarra and Southern Ranges south of Sydney, all popular summer holiday locations.

Shoalhaven mayor Joanna Gash said the area was a "tinderbox".

"Things are not looking real good," she told Sky News.

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.

Also in southern New South Wales, authorities warned that an out-of-control grass fire was encroaching on properties in Brogo, about 160 kilometres (100 miles) south of the national capital Canberra.

"We just looked at each other and said 'We're leaving'," Brogo resident Hallie Fernandez-Markov told AFP from the town of Cobargo, where she was staying with friends after fleeing her guest house.

The popular tourist resort of Kings Canyon south of Alice Springs went up in flames after an out-of-control wildfire spread from the Watarrka National Park, with visitors evacuated by road shortly before the fire hit.

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 degrees in Sydney and hitting 45 in the state's west.

Temperatures have soared so high, and are expected to continue climbing, that the Bureau of Meteorology was forced to add new colours -- deep purple and pink -- to its charts for forecasts above the previous limit of 50 degrees.

Fitzsimmons said while cooler weather had begun to sweep in from the south, seeing temperatures in some areas plummet from 40 degrees to 24 in minutes, the front was moving slowly and would take hours to cover the state.

"We still have hours of hot, dry, difficult conditions ahead of us," he said.

There were also extreme conditions in Victoria Tuesday, with one fire at the town of Kentbruck in the state's southwest burning out of control, with 400 firefighters battling to stop it threatening rural communities.

Wildfires destroyed more than 100 homes in Tasmania over the weekend, and around 40 blazes were still burning across the southern island state but the immediate threat to homes was believed to have passed.

Tasmanian police, who Tuesday continued searching burned out properties, said no bodies had been discovered so far.

Initial reports said as many as 100 people could be missing, but police said there was much confusion about movements during the crisis and there were only "a handful" of people they were trying to locate quickly.

Fires are a regular occurrence in vast and arid Australia, particularly between December and February.

The last four months of 2012 were abnormally hot across the nation and the warm conditions have been exacerbated by very dry conditions due to the delayed start to a weak monsoon.