Sun said temperatures in the fire zone on Sunday exceeded 25 degrees Celsius (77 degrees Fahrenheit), with winds reaching 20 kilometers per hour (12 miles per hour) and humidity dropping to 30 percent.
"It's not as dire as it could be, but it's certainly challenging work for firefighters and expected to get worse," Sun said.
The Defense Department, meanwhile, said it was investigating whether there was any link between the Lithgow fire, which started Wednesday, and military exercises using explosives at a nearby training range on the same day.
"Defense is investigating if the two events are linked," the department said in a statement on Saturday.
Sun said the cause of the fire was also under investigation by fire authorities and would be made public when determined.
Arson investigators are examining the origins of several of more than 100 fires that have threatened towns surrounding Sydney in recent days.
Rural Fire Service Commissioner Shane Fitzsimmons said the weather was forecast to deteriorate further over Monday and Tuesday.
"The worst of that weather will be probably culminating on Wednesday, but (there won't be) much relief in the intervening period," he told Australian Broadcasting Corp.
The wildfires have been extraordinarily intense and early in an annual fire season that peaks during the southern hemisphere summer, which begins in December.
Wildfires are common in Australia, though they don't tend to pop up in large numbers until the summer. This year's unusually dry winter and hotter than average spring have led to perfect fire conditions.
In February 2009, wildfires killed 173 people and destroyed more than 2,000 homes in Victoria state.