Mandatory Credit: NSW Rural Fire Service

Authorities in New South Wales have declared a catastrophic fire danger for greater Sydney, the Illawarra and Shoalhaven regions on Saturday as multiple bushfires rage around Australia's largest city.

Footage shows trees engulfed in massive flames along a road in Gospers Mountain, Wirraba, north-west of Sydney.

"We are asking everybody not to travel on roads anywhere near the vicinity of an active fire unless you absolutely have to," said NSW Premier Gladys Berejiklian, while the state's Rural Fire Service Deputy Commissioner Rob Rogers urged people who live in heavily vegetated areas accessible by single roads to leave.

"Maybe just not necessarily evacuate but be smart and go somewhere else - the beach, shopping centre, somewhere that is not bushfire-prone," he said.

Bushfires have burned 3,700,000 hectares (9,142,899 acres) since the start of the fire season, destroying over 2,500 buildings and killing 12 people.

Mandatory Credit: NSW Rural Fire Service

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