Australian authorities ordered the evacuation of some rural areas of New South Wales as bushfires, fanned by extreme heat and strong winds, raged across the state.

By Sunday afternoon, emergency warnings were issued for five rural areas. People were told to evacuate if they could, or seek shelter and avoid bush or grassland where it was too late to leave.

More than 2,000 firefighters, many of them volunteers, battled 86 fires across the state, with 38 of them still largely out of control.

A heatwave saw temperatures hit record highs in some parts of the state, creating conditions that officials said were worse than those preceding Victoria's 2009 "Black Saturday" fires, Australia's worst bush fire event that killed 173 people.

"This is the worst day we have seen in the history of New South Wales when it comes to fire danger ratings and fire conditions," Shane Fitzsimmons, the state's rural fire chief, told reporters.

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Fitzsimmons said there were unconfirmed reports of homes, farm sheds and machinery being destroyed by fast-moving fires breaking containment lines.

Temperatures climbed above 45 degrees Celsius in some parts. Dry and hot northwesterly winds coming from Australia's desert centre, some up to 75 kilometres an hour, were fanning the bushfires.

There were no reports of injuries, but some firefighters were suffering from heat-related issues.

A 13-year-old boy and a 40-year-old man were charged on Sunday with allegedly starting fires.

A southerly wind change associated with a cold front was forecast to arrive by early evening, the Bureau of Meteorology said.

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Fitzsimmons said the front would eventually offer relief, but would create volatile conditions as it met the northwesterly flow.

Since Friday, heatwave conditions have caused the cancellation of major sporting events and put pressure on the electricity grid.

A paper mill, a water treatment operations centre and Australia's largest aluminum smelter, Tomago, were among businesses forced to halt operations to conserve energy on Friday.