Several Australian bushfires have combined to form a "mega fire" that is burning out of control across a swathe of land north of Sydney, authorities said Friday, warning they cannot contain the blaze.

New South Wales Rural Fire Service deputy commissioner, Rob Rogers said "there are probably more than eight fires in all" that have merged to form what has been dubbed a "mega fire" in an area of national park forest. The blaze was burning across 300,000 hectares - an area roughly 60 kilometres (37 miles) across - within an hour's drive of Australia's largest city, which was again subsumed in a soup of toxic smoke.

"There is just fire that whole way" said Rogers, who added that firefighters could do little more than get any residents out, protect property and hope for an end to fire-friendly dry and windy conditions.

We "cannot stop these fires, they will just keep burning until conditions ease, and then we'll try to do what we can to contain them," he told public broadcaster ABC.

"The best thing we can do is try to protect property and people as much as we can."

Prolonged drought has left much of eastern Australia tinder-dry and spot fires have raged every day for the past three months.

Bushfires are common in Australia but scientists say this year's season has come earlier and with more intensity due to a prolonged drought and climatic conditions fuelled by global warming. Dramatic footage of firefighters running from a wall of fire ripping through the tree canopy above them was captured overnight in Orangeville, less than 100 kilometres west of Sydney.

The local fire and rescue team that shot the images said no one was injured and the vehicles surrounded by the swirling embers also survived the blaze.

"The video was put up to demonstrate why you need to listen to the fire advisory system," the team from Ingleburn fire station said on Facebook.

"If your property is not prepared for the bushfire season and your (sic) not sure you are able or capable of defending your property if a fire approaches you need to leave straight away," they warned.