The NSW National Parks firefighter was hit by a falling tree about 10.45pm on Sunday, while trying to contain a fierce fire near Bulahdelah on the Mid North Coast.
Paramedics tried to help the man but he died at the scene and authorities are trying to contact his family.
Premier Chris Minns said it was a "very sad day" for the firefighting community.
"This is in many ways a dangerous profession and we're deeply indebted to the men and women that devote their time to keeping us safe," he told Channel Nine's Today program on Monday.
Prime Minister Anthony Albanese also offered his condolences to the family and colleagues of the firefighter.
"This terrible news is a sombre reminder of the dangers that our emergency services personnel face to keep our homes and communities safe," he said in a statement.
The NSW Rural Fire Service confirmed 20 homes were destroyed between the Bulahdelah fire and another fire at Koolewong on the NSW Central Coast.
While there are no current emergency bushfire or heatwave warnings, storms and damaging winds are expected to keep fire danger high across much of Australia.
About 60 bush and grass fires were burning in NSW with nine still uncontained on Monday following earlier heatwave conditions, the premier said.
Communities should be prepared for a long and dangerous fire season with forecasts suggesting threatening conditions will be a constant throughout summer.
"If you're asked to go and leave your home, I can promise you firefighters and police are doing that because ... they're trying to save your life," the premier said.
Federal and state disaster assistance funding has been activated across the Central Coast, Mid Coast, Upper Hunter, Muswellbrook, Warrumbungle and Dubbo.
More than 250 firefighters, 50 trucks and nine aircraft battled the Koolewong blaze at its peak where 16 homes were destroyed in an hour.
Trying to encourage people to volunteer in firefighting and emergency response roles for long periods and not only when natural disasters strike is an ongoing concern for the federal government.
"We've got members of our community that run towards (bushfires) to protect not only property but livestock and livelihoods," Emergency Management minister Kristy McBain told ABC radio on Monday.
"They deserve our utmost thanks, but what they really want is more people to join them."
Opposition Leader Sussan Ley also paid tribute to firefighters as she offered her condolences on X.
Meanwhile in Tasmania, authorities confirmed a 700-hectare blaze at Dolphin Sands, near Freycinet National Park on the state's east coast, was contained after overnight rain.
Nineteen homes were destroyed and 14 others sustained some damage in the fire sparked in the seaside community on Thursday.
More than 120 smaller structures and electricity transmissions were also damaged.
The Bureau of Meteorology has issued a severe weather warning for the east of the island, with damaging wind gusts of about 100 km/h expected from late Sunday evening into Monday morning.
About 30 firefighters remain on the fireground continuing to work on extinguishing hot spots.
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