A "mega blaze" raging across a 60km (37 mile) front north-west of Sydney cannot currently be put out, Australian fire officials have warned.
The fire across almost 300,000 hectares (1,150 sq m) is just an hour's drive from the nation's most-populous city.
People who cannot defend their property from approaching fires have been told they should leave immediately.
Since October, bushfires have killed six people and destroyed more than 700 homes across Australia.
The severity of the blazes so early in the fire season has caused alarm, and prompted calls for greater action to tackle climate change.
Fires have also raged across Queensland, Victoria, South Australia, Western Australia and Tasmania.
At one point on Friday, nine fires had been raised to emergency level warnings, although these decreased markedly amid a brief respite in conditions later in the day.
The blazes north of Sydney were sending black fumes across the city, causing a rise in medical problems.
NSW RFS deputy commissioner, Rob Rogers told national broadcaster ABC: "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 said the 60km stretch from Hawkesbury to Singleton was "just fire that whole way".
Video footage from the Orangeville area showed firefighters running from a wall of fire and the Walkabout Wildlife Park has evacuated hundreds of animals.
Fire officials in Ingleburn warned: "If your property is not prepared for the bushfire season and you're not sure you are able or capable of defending your property if a fire approaches you need to leave straight away."
Firefighters from Canada were briefed in Sydney on Friday and will be deployed across New South Wales over the weekend, to be joined by teams from the US.
BDST: 1358 HRS, DEC 07, 2019