The fire is forecast to edge towards the capital on Friday, with a prediction map released Thursday showing it move closer to the southern village of Tharwa.

ACT Emergency Services Commissioner Georgeina Whelan said hot and windy conditions plus a potential thunderstorm were expected over the weekend.

"In a sense the fire will be waking up this afternoon," Ms Whelan told reporters in Canberra on Thursday.

"What we do not want to do is alarm the community, we want you to remain alert."

The fire was about three kilometres away from the small village of Tharwa, south of the capital, and nine kilometres from Canberra's southern outskirts.

Ms Whelan described the fire as a "living being" that was unpredictable and behaving in ways emergency services hadn't seen before.

ACT Emergency Services Minister Mick Gentleman said a state of emergency was likely to be declared - a step up from the current state of alert.

Authorities have warned it is the most serious blaze the capital has faced since the deadly 2003 bushfires, which destroyed almost 500 homes and led to four deaths.

Canberra's police chief Ray Johnson warned locals against "disaster tourism", saying they should avoid the fire area if they did not need to be there.

Police are prepared for evacuations and more evacuation centres would be made available if needed.

Authorities said their main concerns were embers starting spot fires ahead of the main blaze.

The Orroral Valley fire was burning at "advice" level on Thursday and spreading slowly in all directions, with the main fire front 10 kilometres wide and flames one metre high.

Eight firefighting aircraft were fighting the fires, with more than 50 firefighters and 14 vehicles on the ground.

Additional defence reconnaissance aircraft would arrive later on Thursday afternoon.

Ms Whelan said firefighters were working to protect critical infrastructure in the fire area, including a communications tower, a Telstra exchange and solar farm.

Three NSW firefighters were injured after a tree fell onto their truck late on Wednesday night. A fourth firefighters escaped unharmed.

Ms Whelan said the injuries to the three injured firefighters were not life-threatening, after they jumped from their truck moments before the tree collapsed onto it.

All four were transported to Canberra Hospital for assessment but Ms Whelan said they were all doing well.

Queensland firefighters had also joined the fight late on Thursday evening.

The fire was started by a defence helicopter's landing light after it landed in Namadgi National Park while on reconnaissance.

The helicopter was engulfed in 12 seconds, damaging the aircraft before it was able to take off, with all six crew members unharmed.

In good news for firefighters a 400-hectare blaze burning near Canberra Airport had been officially declared extinguished.

Australian Associated Press