The Rural Fire Service said some 85 fires were burning across New South Wales state, with about 20 of them uncontained despite Saturday's cooler weather conditions.
Among the major fires was one burning between the towns of Lithgow and Bilpin some 80 kilometres (50 miles) northwest of Sydney, which intensified after burning through 30,000 hectares (74,000 acres) and reportedly destroying some properties.
The Australian Defence Force said it was investigating the circumstances of the fire near Lithgow, which began on defence land.
"The fire started on 16 October, the same day that Defence personnel were conducting an explosive ordnance training activity," it said in a statement.
"Defence is investigating if the two events are linked.
"Our thoughts are with those who have lost property or whose property is threatened by these devastating fires," it added.
Firefighters are battling bushfires across New South Wales, which could take weeks to fully overcome, particularly with more hot and gusty weather forecast for as soon as Sunday.
One man has already died while trying to protect his home on the Central Coast north of Sydney, possibly succumbing to a heart attack, but authorities are hopeful no other people are unaccounted for in the blazes.
The fires took hold in warm and windy conditions on Thursday and the worst affected areas have been in the Blue Mountains west of Sydney, where some 193 properties were destroyed and 109 damaged in the towns of Springwood and Winmalee.
New South Wales Rural Fire Service Deputy Commissioner Rob Rogers said Saturday's conditions were a "pause" but that the fire perimeter stretched for more than 500 kilometres.
"We're by no means out of the woods," he told national broadcaster ABC.
"It's just calmed down a little bit and obviously we're bracing ourselves for these worsening conditions."
Rural Fire Service Commissioner Shane Fitzsimmons said some of the fires were so big they would take some time to completely extinguish.
"Firefighters will be working on these fires for weeks," he said.
"It's all about reducing the risk of these fires to breach containment lines and run under hotter, drier, windier condition over coming days."
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