Chief Coroner Lorraine Walker will hold an inquiry into the origins of the fire anyway, deciding it was in the public interest.

Ms Walker's inquiry will focus on the 45 minutes it took for the helicopter crew to alert the ACT Emergency Service Agency to the fire's location after their aircraft's landing light accidentally ignited the blaze.

But Attorney-General Shane Rattenbury last month told residents affected by the fire, who had called for an inquiry, that such a probe would not materially advance understanding of what happened.

"Defence has co-operated with the ACT government on all aspects of the Orroral Valley Fire and I do not believe that a coronial inquiry would elicit further information that would be of public benefit," Mr Rattenbury wrote.

Defence would fully participate in the inquiry, a spokesperson said on Wednesday night. The ACT government and its Emergency Services Agency had already confirmed their participation.

Ms Walker said it was in the public interest that all "relevant matters concerning the cause and origin of the fire - and the actions taken to respond to it - are fully considered".

The Canberra Times understands Mr Rattenbury considered the issue of a coronial inquiry for about a month, before deciding the origins of the fire were clearly understood and an inquiry was superfluous.

"Questions about the most appropriate landing place for a Defence asset in an emergency, the focus of Defence on safety of aircrew and ground elements in the immediate aftermath of the incident, the operational procedures Defence had in place at the time of the incident and whether there has been a review of Defence's response to the fire are matters that can only be addressed by Defence," he wrote in June.

Mr Rattenbury also rejected the residents' suggestion there was "significant confusion" about the location of the fire and that fire crews were incorrectly dispatched.

"I am advised by ESA that there was no such confusion about the location of the fire. It should be noted that no vehicle was required to turn around and vehicles were progressing to the vicinity of the fire while the exact location was being refined," he wrote in a letter provided to The Canberra Times.

The fire burned through 80 per cent of Namadgi National Park after it ignited about 1.30pm on January 27.

Defence took 45 minutes to alert the ACT Emergency Services Agency after the fire was started, but ACT authorities say the fire was spotted 19 minutes after it was ignited.

MORE ON THE ORRORAL VALLEY FIRE:

Mr Rattenbury said in his letter contracted ACT aerial resources were dispatched within three minutes of the Mt Tennant fire tower detecting the start of the blaze, and ground crews were at the scene by 2.25pm but could not directly access the seat of the fire.

"The response by ESA and the challenges in gaining access to the fire because of the area in which the fire was burning mean that the timeliness or otherwise of Defence reporting the location of the fire was immaterial to the cause and origin of the fire," he wrote last month.

No homes were destroyed in the ACT as a result of the bushfire, however about a dozen were lost after it skipped the border into NSW.

The ACT coroner cannot investigate what happened in NSW.

The Chief Coroner, Ms Walker, said in a statement the fire burned through the ACT for a month and was a fire of public significance.

"It is appropriate in the circumstances that I hold an inquiry into this significant fire," she said.

The ACT Attorney-General has the power to compel the Coroner to hold an inquiry into a fire if it has destroyed or damaged property.

Hearings for the Orroral Valley fire coronial inquiry are unlikely to be held this year, as the Coroners Court has a significant caseload and limited resources.

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