Bushfires raged across Australia's most populous state Tuesday, fanned by intense heat and high winds in "catastrophic" conditions which have forced hasty evacuations and are threatening homes.

Authorities warned New South Wales state faced one of the highest-risk fire days in its history, and temperatures rapidly climbed above 40 degrees Celsius (104 degrees Fahrenheit).

"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, popular summer holiday locations, with Shoalhaven mayor Joanna Gash saying the area was a "tinderbox".

Authorities have warned that an out-of-control grass fire was encroaching properties in Brogo, about 160 kilometres (100 miles) south of the national capital Canberra, and also in the Kybeyan Valley in southern NSW.

"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 evacuating her guest house.

"It's high winds now, it's really blowy," she said of conditions, adding that temperatures were searing as she drove out along the heavily forested road as firetrucks rushed in to counter the blaze.

A total fire ban is in place throughout the state, while all national parks are closed, with temperatures forecast to peak at 45 degrees. Sydney was by early afternoon sweltering in 42-degree heat.Fitzsimmons said forecasts of hot, windy conditions were proving to be correct.