Fire officials defended Australia's defence department on Thursday after investigators revealed a military training exercise ignited the largest of the wildfires that have torn across the nation's most populous state over the past week.

The Rural Fire Service said Wednesday that one of the more than 100 fires that have burned since last week began at a Defence Department training area as a result of "live ordnance exercises." The fire it sparked near the city of Lithgow, west of Sydney, has burned 47,000 hectares and destroyed several houses, but no injuries or deaths were reported from the blaze.

In a statement, Acting Defence Minister George Brandis said the military was co-operating with investigators.

Rural Fire Service commissioner Shane Fitzsimmons said the defence department's actions were obviously an accident.

"It wasn't deliberate; it was a side effect of a routine activity, it would appear, and clearly there was no intention to see fire start up and run as a result of that activity," Fitzsimmons said. "There is no conspiracy here."

Fire fighting aircraft crashed

Meanwhile, officials said a fixed-wing aircraft helping fight a fire near Ulladulla, south of Sydney, crashed Thursday morning. The fire service said rescue crews were trying to reach the remote crash site.

Fire service Deputy Commissioner Rob Rogers said there was no information about the condition of the pilot, who police said was the only person on board.

Australia Wildfires

More than 100 fires have killed one man and destroyed more than 200 homes in New South Wales state since Thursday. (Dan Himbrechts/AAP/Associated Press)

"Firefighting is an inherently dangerous task, whether you're on the ground or flying through the air," he said. "You've got something that can't be controlled, limited visibility, multiple aircraft working in the area. There's always danger."

The fires over the past week have killed one man and destroyed more than 200 homes in New South Wales state. Sixty-five fires were burning Thursday, with 24 out of control, though all emergency warnings had been lifted.

Investigators are still looking into the causes of the other fires. Some were started by power lines brought down in strong winds, the fire service said. A few appear to have been deliberately lit, and police have arrested several children in connection with those.

The major wildfire threat had passed Thursday, as cooler temperatures and calm winds prevailed and hundreds of residents returned to homes they had evacuated earlier.