The Gospers Mountain mega blaze, which is a combination of several fires, across almost 300,000 hectares is just an hour's drive from the nation's most-populous city, the BBC reported.
People who cannot defend their property from approaching fires have been asked by the authorities to leave immediately.
At 12 p.m. on Saturday, 95 fires were burning, with half yet to be contained, the New South Wales Rural Fire Service (NSW RFS) tweeted.
It said that more than 2,200 firefighters "were out in the field".
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 60 km stretch from Hawkesbury to Singleton was "just fire that whole way".
The blazes north of Sydney were sending black fumes across the city, causing a rise in medical problems.
December 10 is the next big concern, with temperatures inland of Sydney likely to reach above 40 degrees Celsius.
Some firefighters have expressed concern that volunteer numbers might not be enough and that there are inadequate water supplies.
The Bureau of Meteorology (BOM) said that "some fires were too big to put out" while the NSW RFS said late Friday the blazes would only be extinguished "when we get good rain".
Since October, bushfires have killed six people and destroyed more than 700 homes across Australia.
Besides NSW, Fires have also raged across Queensland, Victoria, South Australia, Western Australia and Tasmania.
( With inputs from IANS )