The 51-year-old man was among the first crews to arrive at the house at Grose Vale, west of Richmond, just before 8am on Tuesday.
The firefighter was inside the house when he collapsed, Fire and Rescue NSW Superintendent Adam Dewberry said.
"He was probably struck by a beam and suffered injuries as a result," Supt Dewberry said.
Colleagues from FRNSW and the NSW Rural Fire Service dragged the man from the house and performed CPR until paramedics arrived. He was later pronounced dead at the scene.
The firefighter worked on-call for FRNSW, which is more prevalent in regional areas, and they are notified of calls for help through a pager.
FRNSW had two trucks at the scene from Richmond and Windsor. The organisation is yet to reveal which station the deceased man belonged to.
GROSE VALE | Fire and Rescue NSW firefighter killed at house fire. Fire and Rescue NSW (FRNSW) regrets to advise of the death of a firefighter at a house fire at Grose Vale, north-west of Sydney, this morning.— Fire and Rescue NSW (@FRNSW) The 51-year-old FRNSW on-call fire... https://t.co/BSQRBEA0HXDecember 12, 2023
Supt Dewberry said there was no reliable information to suggest anyone was at the home at the time of the blaze. Firefighters searched the property as part of standard procedure.
He was unaware of major injuries to other firefighters.
The six-bedroom house, flanked by a pool and tennis court, sustained significant damage.
FRNSW said it was working with other authorities to investigate the circumstances of the incident.
It will also provide support to the man's family, crew and colleagues.
Police are investigating and a patrol car is blocking access to the road.
The cause of the fire is yet to be determined.
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