Major lightning storms set off about 100 blazes in South Australia alone, most of which had since burned out, according to the state's Country Fire Service.

Emergency crews also battled scores of fires in the most populous state of New South Wales, some on the outskirts of Sydney, the Rural Fire Service (RFS) said.

More than a quarter of the state was considered at catastrophic risk and lightning strikes set two homes in the city ablaze.

"We have seen more than 80 fires across New South Wales today," said RFS commissioner Shane Fitzsimmons.

"The biggest challenge today and into the weekend is the continuing of this hot air mass dominating much of the state," he added.

Hundred-year records tumbled this week as the south and southeast of Australia sweltered through a heatwave which dried out vast tracts of bush and farmland already in the grip of a decade-long drought.

Flights were delayed due to the unseasonable heat, thunderstorms, strong winds and the effects of smoke on visibility, air traffic officials said.

The first "catastrophic" or "Code Red" warnings -- a new category introduced after February's deadly Black Saturday fires -- were declared in parts of the two states, under which residents are strongly advised to flee their homes.

Code Red conditions are considered on a par with those experienced ahead of Black Saturday, Australia's worst disaster of modern times which killed 173 in the state of Victoria and razed more than 2,000 homes.

"Homes are not designed and planned to withstand conditions typically with this sort of rating," said Fitzsimmons.

"The very hot temperatures we've seen across New South Wales right throughout this last week are simply breaking hundred-year records," he added.

Residents cannot be forcibly evacuated but are strongly advised to leave their property on a Code Red day, which signifies a high risk of death or injury and destruction.

More fires spread across rural parts of Victoria state and have consumed more than 200 hectares (more than 490 acres) of bushland.

Australia is facing its worst fire danger in four years, with hotter and windier conditions and earlier than normal outbreaks forecast, according to government analysis published Friday.

"What we saw on (Black Saturday) was an extraordinary day from a weather point of view. We are starting to see those sort of days more frequently," said fire expert Kevin Tolhurst.

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