A state of emergency has been declared for the ACT, with officials warning weekend conditions could be the worst Canberrans have seen since 2003.
Chief Minister Andrew Barr made the announcement about midday on Friday.
"I made this declaration of the state of emergency today to allow Canberrans, especially those in the south of Tuggeranong, time to prepare for the weekend," 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 ACT had been on a state of alert for some time.
The Orroral Valley fire had grown to 21,500 hectares, or about 8 per cent of the territory's total land mass by 7.30pm on Friday.
The fire was elevated to an emergency warning level, the highest possible warning level.
A statement from the ACT Emergency Services Agency said the Orroral Valley fire was out of control and "creating its own weather pattern".
There was a number of scenarios where this fire may impact on homes in Canberra suburbs.
The ESA earlier warned the Orroral Valley fire could potentially spread to the southern suburbs of Tuggeranong on Sunday under a worst-case scenario.
One forecast fire spread map also shows Tharwa would come under ember attacks on Saturday.
Mr Barr and ACT Emergency Services Minister Mick Gentleman on Friday said the declaration of a state of emergency was the strongest signal they could send to the ACT community to "prepare themselves, and their family, for the worst possible situation".
ACT Emergency Services Commissioner Georgeina Whelan had the ability to coordinate resources across government to respond to whatever risk the territory might face in the coming days.
Ms Whelan said hot and windy conditions, plus a potential thunderstorm, were expected over the weekend.
"Over the next 72 hours, our community will be subject to possible road closures, evacuations, and any other action required to keep our community safe," she said at noon on Friday.
"This is a consequence of the extreme bushfire and heatwave conditions that we may encounter.
"We are concerned the conditions in the ACT will be the worst we have seen since 2003."
Ms Whelan said ground crews only had access to about 1 per cent of the fire spread because of inaccessible terrain.
"We are relying on air assets, however, smoke is making it difficult for aircraft to fly in certain areas over the fire ground," she said.
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A declaration of a state of emergency occurs if the Chief Minister is satisfied that an emergency has happened, is happening, or is likely to happen.
It allows a nominated emergency controller powers to manage the region in the area of emergency.
This includes control over the movement of people, taking possession of any premises, animal, substance or thing in or near the emergency area, and directing people to provide information or produce documents or anything else reasonably needed.
Mr Barr said wildlife had been relocated from Tidbinbilla Nature Reserve, Icon Water had pumped three days of water supplies into city reservoirs, and evacuation centres would be stood up as required.