"People can return to the area. No properties are currently under threat," the Emergency Services Agency said about 9pm.

"Atmospheric monitoring has been carried out in the area and determined air quality to be safe."

The Beard fire formed mid-afternoon after two fires burning at Pialligo merged.

There are unconfirmed reports of building damage. Assessments will commence when it is safe to do so.

Oaks Estate Road between Pialligo Avenue and Railway St was closed as of 9pm.

The fire produced dangerous levels of smoke as it hit the recycling centre in Underwood Street in Beard. The ACT Emergency Services Agency warned this would reduce visibility and air quality.

Those in the area of the smoke were told to stay inside and close all windows and doors and turn off airconditioning and evaporative coolers.

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The ESA website went down about 4.45pm, both in the ACT and across NSW due to a hosting error from website provider Amazon. It was restored by 6.30pm.

The Kallaroo and the Pialligo Redwood Forest fires merged shortly after 2pm.

NSW Rural Fire Service commissioner Shane Fitzsimmons said as of 4.45pm on Thursday he had had no reports of properties lost in the Queanbeyan region.

For those who needed to evacuate the Queanbeyan-Palerang Regional Council opened the Queanbeyan Showgrounds. David Campese Oval was also serving as an evacuation centre.

It was understood that people in some Brindabella offices were being evacuated at 4pm. Sixty cars were being let out in 10-minute intervals.

The Pialligo Redwood Forest started on Wednesday. The fire, which had been downgraded overnight, was taken back to an emergency warning level shortly before noon on Thursday.

The Kallaroo fire started about 11am on Thursday. It is understood to have started at the old Duntroon dairy farm when the wind blew over a tree on to power lines.

The ACT government reinstated the state of alert to the whole of the territory after it had removed it for urban areas on Monday. It will remain in place for Friday and the weekend.

ESA commissioner Georgeina Whelan said those affected by road closures could go to the relief centre at Dickson College.

Buses were also being sent to the Queanbeyan evacuation centre at David Campese Oval to move people to the relief centre.

Ms Whelan said a firefighter in the ACT had sustained minor injuries fighting the fires on Thursday afternoon and was being treated.

The fire had also caused disruptions to rail and air travel.

All passenger flights in and out of Canberra airports were halted because of the fire. Passengers were advised to contact their airline for the latest situation.

The Queanbeyan Railway Station is located in an area under threat as trains between Sydney and Canberra were set for disruption.

A NSW Trainlink spokeswoman said the corridor between Canberra and Goulburn was closed due to bushfire. Coaches will replace rail between Canberra and Goulburn.

Earlier in the day, the 11.05am train from Sydney arrived as scheduled.

Liz Carslund waited for her daughter, Tina to arrive from Sydney. She was slightly worried the train could possibly be halted as smoke and dust was kicked up from the fire.

Shortly after the train arrived, fire activity picked up and the fire was quickly upgraded.

In nearby Oaks Estate, the suburb lost power at about 11.30am, Kate Gauthier said.

She had stayed to defend her property and said other neighbours had also stayed.

"The people who have stayed are just wetting down outside the houses and trees," she said.

"The group of us who stayed all know each other."

Although Ms Gauthier was at her property at 2pm she said she had the car packed and was ready to leave if needed. All roads into Oaks Estate were blocked.

"Luckily for me I can see the bridge out from my house and it's easy for me to get out. The car is completely packed and we already have emergency plans to know what we need to do."

In Pialligo, Gary Brodrick watched the fire start on Thursday morning on a property that has been in his family since the Second World War.

What was once a 140-acre property has dwindled down to just over 10 acres and a bushfire on Thursday threatened what was left.

My Brodrick said the fire started when a tree branch fell off due to strong winds and hit a power pole that encroached on his neighbours property.

"It just took off in a matter of 10 to 15 minutes," he said.

"I was at my truck and then I just looked and there was a fire and it started taking off, I didn't know which way it was going.

"And the wind is really strong... it's a bastard of a day."

There were several crews fighting the fire on Mr Brodrick's property.

"They we're pretty quick to get here," he said.

Mr Brodrick had fielded many calls from friends and family concerned about him.

The fire was burning across the road from Brindabella Park.

Michael had been watching the fire from a balcony on his Brindabella Park office with colleagues.

"As you say it's not something you see everyday so it's probably the worst day you could get a fire out," he said.

"I wouldn't like to be putting a fire out in today's conditions."

Michael was only 30 seconds into his conversation with The Canberra Times when he had to evacuate from the balcony.

The Defence Force was assisting with the fires to a degree, Chief of Defence Angus Campbell said.

"The Defence Force is both assisting to a degree and looking to whether that needs to be reinforced," he said.

"Those fires have, as you know, just emerged in the last couple of hours. I have people who are both involved as persons who need to be moved from areas and office buildings that are potentially in danger, and also those persons who are part of the Bushfire Assist effort. An unfolding situation."

The fire has started during a total fire ban. There is also a severe weather warning for the ACT and region, for damaging winds with peak gusts of up to 90km/h expected.

The winds are coming ahead of a vigorous cold front.

"Over the next 24 hours we can expect cooler temperatures and light winds. This however will not ease for some time so we continue to expect both rural and urban firefighting through the night," Ms Whelan said.

Minister for Police and Emergency Services Mick Gentleman said the state of alert was reinstated following advice from the ESA.

"All Canberrans need to also ensure they have a Bushfire Survival Plan in place and in addition should consider completing an Emergency Grab and Go Booklet which can support evacuation. Both these plans are available on the ESA website," Mr Gentleman said.

"We will continue to assess the situation as conditions change and will make any further amendments to the State of Alert as required."

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Also about 11am on Thursday, firefighters were also called to a tree alight in the middle of the road on the intersection of Bowes Street and Launceston Street in Phillip.

There was another tree fire in Monash about 11.30am.