There was "nothing suspicious" about the blaze that more than 20 firefighters battled in central Wagga at the weekend.
NSW Fire and Rescue Wagga zone commander Daryl Manson said they were called to the Docker Street home just after 5pm on Saturday.
"We were faced with an old-style building made of brick and an iron roof heavily involved in a fire," he said.
"It took us a substantial amount of time to bring it under control, and that was due to a couple of factors, including trouble isolating the power and the structural integrity of the building.
"The roof had come down, and some of the brick walls were unstable."
Inspector Manson said the single occupant had evacuated, and their next priority was to protect the buildings on either side.
"We commenced a defensive attack, and neither properties on either side were damaged," he said.
"It took a little over an hour to extinguish.
"The home has been severely damaged by fire."
Inspector Manson said there was nothing suspicious about the blaze, and the occupant had informed them that he had been using a bar heater and there were some items near it.
"He had left the room for a few minutes, and when he came back, the room was heavily involved in fire," he said.
"It's a timely reminder to be safe, and we can't emphasise it enough - have a working fire alarm."
Inspector Manson said the incident had been another fantastic effort by emergency services, all of which had worked together.
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