Australia bushfires rage in 'catastrophic' conditions

Authorities have warned that New South Wales state faces one of the highest-risk fire days in its history, with temperatures rapidly climbing towards 40 degrees Celsius.

"We've got 100 fires on the books, we are now dealing with just over 20 fires that remain uncontained," New South Wales Rural Fire Service commissioner Shane Fitzsimmons told reporters in Sydney.

The greatest risk is in the Shoalhaven, Illawarra and southern ranges south of Sydney, with Shoalhaven mayor Joanna Gash saying the area was a "tinderbox".

Gash told national television that local residents were "preparing for the worst but hoping for the best" with winds expected to pick up later Tuesday as the temperature soared.

Near Brogo, in the Bega Valley in the state's south, authorities said an uncontained blaze was posing a threat to homes and that residents were urged to evacuate.

A total fire ban is in place throughout the state, while all national parks are closed, with temperatures forecast to peak at 45 degrees, and Sydney set to swelter in 43-degree heat.

Fitzsimmons said forecasts of hot, windy conditions were proving to be correct.

"It is going to be a long hot day for firefighters, it is going to be a long hot day for communities," he said, adding that Sydney itself would likely bake at more than 30 degrees until after midnight.

Earlier he said the state was "shaping up for one of the worst fire danger days on record".

"You don't get conditions worse than this, we are at the catastrophic level and clearly in those areas leaving early is your safest option," he said.

Prime Minister Julia Gillard said she was being briefed continuously and urged caution.

"The word catastrophic is being used for good reason," she said. "So it is very important that people keep themselves safe, that they listen to local authorities and local warnings.

"This is a very dangerous day."

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.

There were also extreme conditions in Victoria Tuesday, with one fire at Kentbruck in the state's southwest doubling in size overnight with about 500 firefighters and 10 aircraft battling to stop it threatening rural communities.

Wildfires destroyed more than 100 homes on Tasmania over the weekend, and around 40 blazes were still burning across the southern island state on Tuesday, according to local reports.

A handful were out of control and Tasmanian Fire Service station officer Phil Douglas said authorities were ready for possible evacuations.

"It's something we are mindful of, that it may happen, so we're prepared for it," Douglas told reporters.