A fire started by a defence helicopter landing in the Namadgi National Park south of Canberra has been downgraded due to easing conditions after earlier becoming the city's "most serious" bushfire threat since 2003.

Key points: Residents in Tharwa Village, Boboyan Road, Apollo Road and Top Naas Road have been told to remain vigilant

Residents in Tharwa Village, Boboyan Road, Apollo Road and Top Naas Road have been told to remain vigilant Those in Banks, Gordon, Conder, Calwell and Theodore have been told to monitor conditions

Those in Banks, Gordon, Conder, Calwell and Theodore have been told to monitor conditions Chief Minister Andrew Barr earlier said the fire posed the greatest risk to the ACT since the 2003 bushfires

The Orroral Valley fire had been growing at a rate of more than 400 hectares per hour, and was more than 7,900 hectares in size at 7:30pm on Tuesday before it was downgraded to watch and act about 10:20pm.

There is currently no threat to Canberra suburbs, but there were fears the blaze could spot as close to a kilometre from the city's southernmost suburb Banks after it started spotting around the township of Tharwa south of the city.

ACT Chief Minister Andrew Barr said the fire posed the greatest risk to the ACT since the 2003 bushfires, which killed four people and destroyed hundreds of homes.

"This is the most serious situation we have faced since the 2003 bushfires," Mr Barr said.

Earlier in the evening, winds were gusting north-easterly at speeds up to 40km/h, according to ACT Emergency Services Agency (ESA) commissioner Georgeina Whelan.

"The weather this evening is not favourable for firefighters," Ms Whelan said.

She said she expected the fire to burn in the national park for several weeks.

Defence helicopter likely caused fire

The Department of Defence has admitted the fire was likely caused by one of its own helicopters, after one of its landing lights created enough heat to set the grass on fire.

"The fire started during routine aerial reconnaissance and ground clearance work being conducted in the area in support of our local firefighters and authorities," Emergency Services Minister Mick Gentleman said on behalf of the Department of Defence.

Defence said one of its MRH-90 helicopters, normally used for troop transport, likely caused the fire as it was landing. ( ADF )

Lieutenant General Greg Bilton said the helicopter was using the lights to help it land in smoky conditions, but the heat set a fire that grew rapidly and damaged the aircraft.

He said defence was investigating the issue but operating procedures would be changed so that the landing lights were not used in extreme conditions.

It is reportedly the first time a fire has been started by a helicopter landing light.

Day that began with calm deteriorated before conditions eased

The day began with emergency services assuring residents in the nearby township of Tharwa that there was no risk to the community of 80.

The township was later told it was too late to leave, and that they must seek shelter.

As conditions eased in the evening, residents in Tharwa Village, Boboyan Road, Apollo Road and Top Naas Road were told to remain vigilant, with officials saying it would still be in their best interests to not enter or return.

Those in Banks, Gordon, Conder, Calwell and Theodore were told to continue to monitor conditions.

The Orroral Valley fire is sending thick smoke over Namadgi Park. ( ABC News: Tamara Penniket )

Residents in Banks, Conder and Gordon had earlier been urged to enact their bushfire plans and were doorknocked before the fire was downgraded, though officials stopped short of issuing an evacuation warning.

The smoke has brought back "anxious" memories of the 2003 Canberra bushfires, according to Ms Whelan, who said the ESA moved forward community meetings in response.

Tharwa resident Dick Wells said locals had been preparing for the worst.

"When it blew up [on Monday] afternoon it was pretty alarming because we saw the big cloud and it was running hard," he said.

"Most of us are pretty exhausted, preparing for about the last six weeks, and in some ways it's a good advantage, but fatigue comes in."

Heatwave will bring worse days for fire conditions

Smoke from the bushfire as seen from homes in Canberra's south. ( Supplied )

The ACT has already been placed under a total fire ban for the remainder of January.

The ESA said an upcoming heatwave, which will bring predicted days over 40 degrees Celsius, will make it very difficult to control the fire.

Much of it is burning in some of the most inaccessible terrain of the rugged Namadgi National Park.

Rural Fire Services Commissioner Joe Murphy said it was also burning quickly and erratically.

"This is not a fire that is operating under normal rules," he said.

"We're not going to get it out [on Tuesday], or any time soon. This is going to take significant effort."