The ACT is in the most dangerous situation since the bushfires in 2003 when four people died and 470 homes were destroyed, according to Chief Minister Andrew Barr.
He urged Canberrans to "prepare for the worst case scenario".
He was speaking alongside grim-faced chiefs of the emergency service, military and police who were preparing for a long night of effort as the Orroral Valley fire in the Namadgi National Park roared out of control.
Mr Barr's tough warning was pitched to prevent panic but also to be strong enough to instill in Canberrans an urgent need to plan.
"Be alert not alarmed", Emergency Services Commissioner Georgeina Whelan said.
"Conditions are very dangerous," she added, warning that the emergency services were stretched.
People shouldn't expect a fire-fighter at every door, as she put it. "I will not be able to put a fire truck on every corner."
Police and ACT government workers were preparing to knock doors and advise people in Canberra's southern suburbs on Tuesday evening.
The ACT was not under immediate threat from the fire but spot fires ignited by embers flying, in some cases, five kilometres were getting close.
Details of an evacuation plan emerged.
Commissioner Whelan said her colleagues in emergency services were working with people in the transport department of the ACT government to make sure that there were enough buses to ferry people to safety.
She urged every person in Canberra - and not just those closest to the fire - to draw up their own individual plans for what they would do in the worst case.
"We aren't catastrophising," she said, "and we don't want to alarm the community but we want the community to remain alert and to think about how they can help the Emergency Services Agency to help them."
She said the authorities were putting together plans "across the scenarios and one of those scenarios is that the southern suburbs will have to be evacuated."
In the wake of the big bushfire flare-up on New Year's Day, the ESA was working on the plan which has been drawn together by different agencies, including the police, emergency services and transport departments.
An ESA spokeswoman said in January that the aim was "to ensure a co-ordinated approach being adopted in the event an evacuation was necessary".
"In the event of an evacuation, Canberrans would be informed through the bushfire warnings system of the need to evacuate, which evacuation centres have been opened and the appropriate roads to take to leave an area.
"There are a number of colleges across the ACT that can be established as evacuation centres if required. Centres will be pet friendly and accessible for people with a disability."