Rural properties are under threat as the out-of-control Orroral Valley bushfire burns south of the capital, which is facing its worst conditions since the deadly 2003 fires.

ACT Emergency Services Commissioner Georgeina Whelan on Friday night said conditions remained "erratic".

"Our urban strike teams will remain in place at south Tuggeranong this evening for rapid response should the need arise," she said on Friday night.

"Currently fire conditions and fire behaviour remain volatile."

An emergency warning was issued on Friday afternoon for the Orroral Valley fire in the Namadgi National Park after a state of emergency was declared across the ACT.

Ms Whelan said the fire could ease later on Friday night before conditions deteriorated again on Saturday morning.

"It's been a tough day. Tomorrow, unfortunately, it doesn't get much better."

People in the area are being told to leave immediately on the road towards Canberra and the village of Tharwa.

ACT Chief Minister Andrew Barr declared the state of emergency on Friday with the fire forecast to move north towards the city over the weekend.

"The ACT is now facing the worst bushfire threat since the devastating fires of 2003," he told reporters.

"The combination of extreme heat, wind and a dry landscape will place suburbs at Canberra's south at risk in coming days."

The bushfire has burnt eight per cent of the ACT.

Canberra is facing two consecutive days of 41C maximums on Friday and Saturday with storms and 35C predicted for Sunday.

Evacuations and road closures are possible as the fire edges closer to Canberra.

Saturday is expected to be the most dangerous day with "textbook" fire conditions.

"What are the 10 things contributing to having a really, really dangerous fire? They are lining up," Ms Whelan said.

Heat, the fire index rating, the territory's topography and fuel loads are among the leading causes of concern.

Fears are growing the fire could break containment lines with ground crews having access to just one per cent of the spread due to inaccessible terrain.

"It has been unpredictable. It remains challenging. It is difficult to access. This fire could create its own weather system," Ms Whelan said.

"If all of what I have just outlined occurs, there is a chance this fire could break containment lines."

Smoke is making it difficult for aircraft to fly over certain areas of the fire ground.

Mr Barr said the declaration was made to allow Canberrans, especially those south of Tuggeranong, time to prepare for the weekend.

"The state of emergency is the strongest signal we can send to the ACT community that they must prepare themselves and their families," he said.

"I understand the anxiety this announcement will cause, especially for those who lived through the 2003 bushfires. This is the first time a state of emergency has been declared since that tragic event."

The 2003 fires killed four people and destroyed more than 500 homes.

Australian Associated Press