The Green Wattle Creek fire in Sydney's south west started on November 27, 2019, and claimed the lives of two NSW RFS volunteers, Geoffrey Keaton and Andrew O'Dwyer on December 19.

The inquiry before NSW State Coroner Teresa O'Sullivan heard from three witnesses that lightning strikes were the likely cause of the fire, while a back burn on December 14 broke containment lines and became a threat to the Balmoral region.

Detective inspector Clements, the officer in charge of the investigation, said he determined that the Green Wattle Creek fire had been caused by lightning strikes in the Burragorang Valley.

"Those lightning strikes matched up to areas where the fire was first identified that particular area was dense bushland, kilometres away from any road or track, or a particular area.

"There was no sign of any man-made object or anything other than natural bush."

Detective senior constable Thomas Dodds also gave testimony.

Detective senior constable Dodds was part of Strike Force Toronto to provide support and to review the unfolding bushfire crisis over that bushfire season 2019 2020.

"In conducting my review, Strike Force Toronto provided remote access to the RFS systems, and were able to review five progression maps and other information that was obtained in their system," he said.

"In doing that, I reviewed lightning strike data effectively to assist Detective Sergeant Clements in identifying the cause and origin.

"Overwhelmingly, evidence suggested lightning as the cause."

Detective senior constable Dodd said they were able to determine by GPS coordinates, three individual strikes that "almost perfectly matched up to the identified hotspots."

"The lightning strike data that I had arranged 48 hours from the 25th to the 27th and took into account all of New South Wales so that was more than 82,000 lightning strikes.

"I don't have the exact number in front of me that relates to the specific area around the point of origin but I would say it was more than 500 strikes."

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Assisting counsel Adam Casselden's opening statements outlined the impact of the fire, which burned more than 278,000 hectares stretching from the Jenolan Caves in the northwest, as far as Taralga in the south-west and as far as Mittagong, Braemar and Bargo in the South East.

The fire also claimed the lives of two RFS volunteers Geoffrey Keaton and Andrew O'Dwyer.

"The loss caused by the fire was catastrophic on several levels," he said.

Mr Casselden's statement and testimony from the witnesses also detailed how a planned back burn broke lines and threatened the village of Balmoral.

"The burn started progressing towards Balmoral by 12:45 pm," Mr Casselden said.

"The backburn had crossed the southern containment line, and the fire took hold over the coming days and continue to spread towards Balmoral.

"The fire came to a tragic climax on Thursday, December 19, 2019.

"Tragically it was during the response to this part of the fire that Jeffrey Keaton and Andrew O'Dwyer lost their lives."

Detective Inspector Clements said the investigation also looked at the back burn.

"Investigation concluded that the backburn inadvertently extended beyond its planned southern termination point due to a lack of clarity on containment lines, backburn alignment, and status within the incident action plan operational mapping.

"Witnesses described a progressive approach to detailed planning on the Green Wattle Creek fire. Under this approach, an agreed strategy for containment was developed in consultation with local groups, local group officers, brigades, and land management agencies.

"Divisional commanders and local brigades were tasked to progressively work along the conceptual containment line, undertaking reconnaissance of options, engaging with landholders agreeing on detailed alignment tasks to construct enhanced lines and then backburning from containment lines once completed."

Detective inspector Clements said a detailed alignment was determined and mapping had been updated, however, the investigation found that in the days before backburning, the process appeared to have broken down.

"The detailed backburn alignment identified based on reconnaissance undertaken on December 11 was not mapped, leaving the conceptual backburn alignment from the overarching strategy within the operational mapping and incident action plans with vague non-specific tasking," he said.

"The alignment did not appear to be constructed before December 14 despite an understanding that it had been.

"There seems to have been an assumption that containment lines had been completed, as there were no documented allies to provide a common reference point."

On December 19, detective inspector Clements was in the Bargo vicinity for traffic control and evacuation coordination.

He described the conditions on that day as more than 40 degrees.

"The smoke cover was intensely thick and driving between Thirlmere and Buxton that particular day. I had a hard patrol vehicle in front of me that had the red lights flashings and sirens, and they were difficult to see and the car was only a short distance in front of me," he said.

"That's how thick the smoke was during that period.

"There were trees that were on fire. However, the smoke was so thick that you really couldn't see damage to the trees."

NSW Rural Fire Service investigator Andrew Sweeney also testified lightning strikes were the likely cause.

During a helicopter review of the impacted area, Mr Sweeney said he saw several trees that had been affected by lightning and identified some of the telltale features that helped him conclude the Green Wattle Creek fire had been started by lightning.

"You could clearly distinguish that that top canopy and top side of the tree had been broken off and was trailing down into the bottom area of the tree from the indicators of a lightning strike," he said.

"The location of the fires was very inaccessible, there were no trails that would lead to that location.

"There were no powerlines through the immediate area, and no rail lines and no equipment use as there were no fire trail roads within the area at all."

He said he was comfortable concluding that the fire was ignited by a lightning strike.

Senior constable Dodds responded to questions about the origins of the fire, saying it was in a "borderline inaccessible area, definitely inaccessible for a road vehicle," he said.

"Borderline inaccessible tracking, unless following the river, I can see that looking at the suspected area of origin is four and a half to five kilometres from the nearest fire trail. And there are no listed campsites in that area."

He said he was satisfied to say the fire was ignited by lightning strikes.

The inquiry into the death of Geoffrey Keaton and Andrew O'Dwyer will be held on Wednesday and Thursday of this week.