Australia's most populous state is paralysed by "catastrophic" fire conditions amid souring temperatures, while one person died as wildfires also ravaged the country's southeast.

"Catastrophic fire conditions are as bad as it gets," New South Wales Rural Fire Services Commissioner Shane Fitzsimmons told reporters.

Holiday-makers have been urged to avoid "non-essential" travel, as several roads - including the Hume and Princes highways - have been closed to motorists because of the fires.

"Given we have a landscape with so much active fire burning, you have a recipe for very serious concern and a very dangerous day."

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Areas in western Sydney were forecast to hit 47 degrees Celsius (115 Fahrenheit) while the west of the state also sweltered temperatures of the high 40s.

A cooling change is expected to move through New South Wales late today, although authorities warned that strong winds could push fires in dangerous new directions.

New South Wales is in a seven-day state of emergency as around 2000 firefighters battle 100 wildfires.

Two firefighters died Thursday battling blazes southwest of Sydney.

Geoffrey Keaton, 32, and Andrew O'Dwyer, 36, were in a truck convoy southwest of Sydney when a tree fell and caused the vehicle to roll off the road.

Authorities confirmed Saturday one person died and 15 homes were destroyed in South Australia as a wildfire ravaged the Adelaide Hills on Friday, just 40km from the state capital of Adelaide.

Another person was critically injured after fighting to save his home from the fires.

It follows the death of a 24-year-old man in a road crash in South Australia on Friday, which sparked a fire in the area of the Murraylands.

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Authorities said 23 firefighters and several police have also suffered injuries, as more than 40,000 hectares burnt across South Australia.

"It is going to be a real scene of devastation, especially for those people in the Adelaide Hills who have been most affected," South Australia Premier Steven Marshall said.

"We know that in addition to the buildings and vehicles lost there are very significant losses in terms of livestock, animals, crops, vineyards."

The annual Australian fire season, which peaks during the summer, started early after an unusually warm and dry winter.

Around 3 million hectares of land has burnt nationwide during a torrid past few months, with nine people killed and more than 800 homes destroyed.

The devastation has put pressure on Prime Minister Scott Morrison, who has copped criticism for going on a family holiday in Hawaii during the wildfires crisis.

He apologised on Friday for "any offence caused to any of the many Australians affected by the terrible bushfires by my taking leave with family at this time".

Fire and Rescue personnel run to move their truck as a bushfire burns next to a major road and homes on the outskirts of Bilpin on December 19. Photo / Getty
Fire and Rescue personnel run to move their truck as a bushfire burns next to a major road and homes on the outskirts of Bilpin on December 19. Photo / Getty

Morrison said he would cut short his vacation and was expected to return to Sydney on Saturday, where he is due to visit the Rural Fire Service headquarters.

Debate has reignited on whether Morrison's conservative government has taken enough action on climate change.

Australia is the world's largest exporter of coal and liquefied natural gas.Fatih Birol, International Energy Agency executive director, believed Australia had missed opportunities to mitigate the impact of coal.

"I find the Australian energy debate far too emotional, far too nervous and far too hot. It is hotter than the climate change itself," he told The Sydney Morning Herald and The Age.

Protesters on Thursday camped outside Morrison's Sydney residence demanding urgent action on climate change.

Morrison, who critics have deemed a climate change skeptic, conceded earlier this month that "climate change along with many other factors" contributed to the wildfires.