Firefighters in the Australian state of New South Wales are bracing for "catastrophic" fire conditions as temperatures well above 40C and strong winds are set to fuel more than 100 fires burning across the state.

Authorities asked people to delay travel, at the start of what is normally a busy Christmas holiday period, warning of the unpredictability of the fires as winds of up to 70km/h were set to fan flames through the middle of the day.

"Catastrophic fire conditions are as bad as it gets," New South Wales' (NSW) Rural Fire Services Commissioner Shane Fitzsimmons told journalists.

"They are the very worst of conditions. Given we have a landscape with so much active fire burning, you have a recipe for very serious concern and a very dangerous day."

Greater Sydney and two surrounding areas were rated as catastrophic today, and other areas were at extreme or very-high fire danger ratings.

Close to 10,000 emergency personnel will be working across NSW today, which the NSW Minister for Police and Emergency Services David Elliott said was likely the largest emergency deployment the state had ever seen.

A southerly wind change is expected late afternoon.

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It is forecast to bring winds of up to 90km/h, which Mr Fitzsimmons said would initially worsen fire conditions before leading to a dramatic drop in temperatures.

The Gospers Mountain mega fire, which has already burned almost 1.1 million acres to the northwest of Sydney was upgraded to emergency status.

The death of two firefighters on Thursday night when their fire truck was struck by a falling tree as it travelled through the front line of a fire brought the wildfires death toll in New South Wales to eight since the start of October.

Shortly after the two deaths were announced, Prime Minister Scott Morrison issued a statement saying he would return as soon as possible from a family holiday in Hawaii, a trip that had drawn sharp criticism as the wildfires crisis deepened.

Australia has been fighting wildfires across three states for weeks, with blazes destroying more than 700 homes and nearly three million acres of bush land.