During a video of his formal police interview shown in the Queanbeyan Local Court on Thursday, Leslie Windser, now 73, could offer detectives no explanation for how the fire started on a property fenceline, adjacent to that of his then-neighbour Neoni Banks, just off the Kings Highway outside Bungendore on December 16, 2019.

However, he admitted to being there at the time.

Mr Windser has pleaded not guilty to charges of intentionally causing a fire and being reckless as to its spread, and causing or setting fire to the property of another.

The bushfire was triggered at the height of the awful 2019-20 Black Summer, with firefighters desperately attempting to quell the North Black Range wildfire near Braidwood, about 11kms away.

Earlier, the hearing was told how a fire-spotting aircraft conducting an aerial line scan on the North Black Range also detected another thermal source some distance away.

The scan detected an L-shaped fire line of 47-metres long and more than four metres wide. In fact, the evidence given alleged that the fire was being lit as the line scan was being conducted.

Fire units, many diverted away from the North Black Range bushfire, were called to immediately respond to this new threat and successfully brought it under control, but not before it had threatened homes and burnt through 62 hectares of private property.

In his formal police interview, Mr Windser described how he had been at the bottom of his property assessing what repair work would be needed on a floodgate across a creek which borders his property with that of Ms Banks when saw what he described as a "dust spiral" which changed colour.

He recognised the nature of the threat and said he "bolted" back up to his house to get some water but by then the fire had "taken off".

The impact of the bushfire was traumatic for Ms Banks, who lost three quarters of her farmland and nearly her home to the blaze which also heavily scoured the property of another neighbour, Barry McRoberts. Ms Banks has since been forced to sell her property for well under its market value and has left the area.

When questioned specifically about the cause of the fire, Mr Windser told police: "I have no idea".

When Mr Windser was told by detectives that expert investigators were in no doubt the origin of the fire was under his fenceline, the accused said he was "just flabbergasted".

"I wish I could say I was doing something stupid, but I wasn't," he said in the interview video.

During the course of the hearing before Magistrate Roger Clisdell, several NSW Rural Fire Service experts, including an ember expert, a bushfire analyst and experienced bushfire investigator Stephen May, had provided evidence which had scotched the theory that an ember had travelled through the air from the North Black Range wildfire and triggered the Kings Highway blaze.

Defence barrister Paul Winch tendered to the court a report from Sydney-based forensic fire investigator Vithyaa Dayalan, but her expertise in determining the cause and origin of bushfires quickly was challenged by public prosecutor Christopher Davis.

After hearing the arguments, Magistrate Clisdell ruled that he would allow Ms Dayalan's report to be tendered but said he was uncertain as to the weight he could give to it, identifying her level of bushfire investigation expertise was "limited, compared with that of the other experts" who had provided testimony previously.

Given Ms Dayalan had not been able to review Mr May's exhaustive investigative report, Mr Clisdell said that in fairness he would need to adjourn the hearing again. It is due to return to court on April 28 next year.

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