Bungendore man Leslie William Windser found guilty of setting fire to another's property during Black Summer bushfires 2019

Leslie William Windser fronted the Queanbeyan Local Court on Thursday for a hearing after pleading not guilty to intentionally causing fire and being reckless about its spread.

He also contested the backup charge of causing fire to another's property.

The prosecution case was that Windser, in his mid-70s, in December 2019 used a gas-powered weed-torch device to burn tussocks along two fence lines at his property just off the Kings Highway near Bungendore.

It was the ignition to the second line that had escaped and burnt others' lands, leaving extensive damages.

At the time, bushfires were raging across NSW with the closest one about 10km away in the North Black Range near Braidwood.

The circumstantial case was that all other sources of ignition - including an ember attack from bushfires as far as 44km away, lightning, cigarettes and others on the site - were eliminated.

The prosecution called on a number of experts, including fire investigator Steven May, who has looked at about 140 bushfires.

Mr May, who attended the site, concluded that a fire was lit to clear vegetation because of the pattern of burns and the fire's linear path.

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Windser in his evidence gave multiple versions of the incident, including that he saw some sort of whirling phenomenon and a puff of smoke.

He then yelled "fire in the creek" to his wife before he raced to get some buckets of water to try to contain the fire, which was ultimately put out by the NSW Rural Fire Service.

In another version, he said he was not home at the time.

Prosecutor Christopher Davis said it was "hard to say" which version to believe and the defence about an ember attack being the cause was a "magical ember" that had travelled tens of kilometres to burn the tussocks.

Defence lawyer Paul Winch argued the prosecution could not prove it was Windser who lit the fire and that the circumstantial case was not strong.

Mr Winch said his client had offered police to search his premises for such a gas-powered device as alleged.

"Nothing of that kind of item was looked for or found on his property," Mr Winch said.

"Nor in the light of all of the evidence, is it conceivable given the evidence including today that Mr Windser did use or own such a device."

The defence also argued ignition was possible from an ember attack and relied on its own expert witness.

Vithyaa Dayalan, general manager and senior fire investigator at Fire Forensics, said she "can't rule out the fact that there's a possible ember attack".

"Just because there is a straight line does not mean it started in a linear fashion," she said.

During cross examination, however, she admitted she did not visit the site and only viewed photographs supplied to her.

The defence also called on five witnesses to give evidence about Windser's good character, which included him being fire conscious because of his meticulous and immaculate cleaning of his property.

Magistrate Roger Clisdell found Windser not guilty of the primary charge and guilty of the backup one.

"I reject that the fire could've been started by an ember attack," Mr Clisdell said.

"Anyone who has ever been subjected to an ember attack knows that it's wide and varied," he said.

The magistrate, who also spoke about his personal experiences of bushfires having lived in rural NSW since 1983, said he was satisfied Windser was responsible for the fire as he worked on his fence lines that day.

"He has been meticulous about getting rid of tussocks from his property and expressed some annoyance about the fact that [a neighbour] had let that particular weed run rampant," Mr Clisdell said.

"This was a fire started by human intervention and only one person was working at the site."

As for the defence argument about police not searching the offender's premises, Mr Clisdell said Windser made the offer more than three months after the fire.

"There is no other reasonable hypothesis...it doesn't really matter what the police didn't find," he said.

"A bloke who robs a bank and tells the police to come search his property three months later for the firearm generally won't have the firearm sitting in his kitchen."

Mr Clisdell acknowledged the offender's good character and lack of criminal history.

"Unfortunately, that does not mean you can't make a mistake from time to time," he said.

"He didn't set out to burn anyone's property down ... effectively this is negligence in taking a risk. He made a dreadful mistake."

Windser was convicted and fined $2000 to be paid within 28 days. He declined to comment outside of court.

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