Defence and the ACT's Emergency Services Minister Mick Gentleman confirmed on Tuesday evening the Orroral Valley fire burning south of Canberra was likely sparked by the landing light of an MRH-90 Taipan helicopter.

Chief of joint operations, Lieutenant General Greg Bilton said a team of six soldiers landed in the remote area of the park about 1pm on Monday in order to clear landing zones for aircraft.

Soldiers had been sent into Namadgi for at least four days to undertake similar missions.

Those landing zones would have enabled emergency services to send strike teams in to deal with fires, he said.

Instead, they ended up accidentally starting what ACT Chief Minister Andrew Barr has called the "most serious" bushfire threat to Canberra since 2003.

"When the helicopter landed yesterday afternoon we believe that the landing light - which is lit on the aircraft as a safety precaution when you're flying in difficult circumstances, such as a very smoky environment - we believe that created enough heat to set the grass on fire," Lieutenant General Bilton said.

From there, the fire spread rapidly.

It is now growing at 400 hectares an hour, ACT ESA Commissioner Georgeina Whelan said on Tuesday.

Even the aircraft itself was damaged by the fire although managed to make it back to Fairbairn, Lieutenant General Bilton said.

In a statement, the Defence Department said it was "deeply regrettable" that its operations had likely started the fire.

"Over the past few weeks, ADF aircraft have been an important part of fire prevention and control activities, in close working partnership with the ACT Emergency Services Agency," the department said.

"We continue to work closely with the ESA to provide personnel and assets to help contain this bushfire.

"The ADF's support to state and territory authorities will continue for as long as needed."

Mr Gentleman said it was the first time a fire had been caused by a helicopter landing light.

The Defence Force has now changed its operating procedures and will not use those landing lights when setting down in extreme weather conditions.

It is also also continuing to investigate how exactly the landing light caused the fire.

"We acknowledge the safety aspect but it's important we're able to continue operations in support of the emergency services," Lieutenant General Bilton said.

Mr Gentleman said despite this incident, the ACT government was grateful for the help of the Defence Force since the territory declared a state of alert early in January.

Defence Force personnel were helping to fight the fire, while helicopters are still being used for fire spotting and fire mapping.

Extra helicopters have been deployed to search ahead of the fire, to ensure the area has been fully evacuated and people have observed the park's closure, the Defence Department said in a statement.

Uniformed Defence Force personnel also helped firefighters and police to doorknock homes in Tharwa, Banks, Gordon and Conder to ensure residents are prepared.

"I'm sure the Canberra community shares our gratitude that none of our defence staff were injured during this incident," Mr Gentleman said.