The latest death came after two people were killed Wednesday when an "Armageddon-like" blaze swept through parts of South Australia, leaving insurance claims of Aus$61 million (USD 44 million) in its wake.
The 48-year-old firefighter collapsed and died after he and New South Wales state Rural Fire Service (RFS) colleagues were tackling a blaze near Cessnock, north of Sydney, late Thursday.
"It's absolutely tragic, someone goes out for the want of nothing -- for free -- to make a difference in their community and haven't been able to make it home," RFS Commissioner Shane Fitzsimmons told reporters.
Fires are burning around New South Wales but milder conditions today reduced the threat, while in South Australia the weather is expected to ensure fires remain contained.
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Damage assessments were being made around the town of Pinery, 70 kilometres (45 miles) north of Adelaide, where a blaze claimed two lives and injured 19 others this week, the Country Fire Service (CFS) said.
At least 35 homes were destroyed and a further 42 were damaged by the fire, the service said.
The Insurance Council of Australia said that, as of today, it had received 415 claims resulting from the South Australian blaze, the cause of which is still unknown.
"These fires have had a devastating impact on the farming community with crop, stock and commercial assets making up the overwhelming bulk of the losses so far," chief executive Rob Whelan said.
Bushfires are common in Australia's hotter months. "Black Saturday", the worst firestorm in recent years, devastated southern Victoria in 2009, razing thousands of homes and killing 173 people.