The Orroral Valley fire could potentially spread to the southern suburbs of Tuggeranong on Sunday under a worst-case scenario, the ESA warns.
The ACT Emergency Services Agency said the fire was travelling in an easterly direction towards Top Naas Road.
The fire crossed Boboyan Road at the Glendale Depot.
The fire is also moving north and is one kilometre south of Corin Road.
The fire is out of control and properties may be impacted by embers.
Leaving immediately is the safest option.
The safest route out of the area is north on Naas Road towards Canberra.
Those living in Tharwa, Banks, Gordon and Conder as well as surrounding areas have been urged to monitor conditions.
A forecast fire spread map issued by the ACT Emergency Services Agency on Friday also shows Tharwa would come under ember attacks on Saturday.
Firefighters are predicting the 19,000-hectare blaze south of Canberra will creep closer to the city as temperatures spike and winds increase Friday.
ACT Chief Minister Andrew Barr enacted a state of emergency for the territory at midday.
The ESA advises people in the areas of Boboyan Road, Top Naas Road, Apollo Road and Tharwa Village could be affected by the bushfire. People need to be ready to follow bushfire survival plans.
ACT Emergency Services Commissioner Georgeina Whelan said hot and windy conditions plus a potential thunderstorm were expected over the weekend.
"If wind conditions prove to be challenging, we could see suburbs such as Conder, Banks and Gordon come under attack from ember attacks on Saturday afternoon," Ms Whelan told ABC radio on Friday morning.
"These are the areas we have been doorknocking for several days now. What people will see is potentially spot fires continue to be ignited forward of the main body of the fire."
The fire is nine kilometres from the capital's southern outskirts and three kilometres from the village of Tharwa.
As of 3pm on Friday, it was 19,618 hectares and at an emergency warning level.
Access to the firefront for direct firefighting operations has been restricted due to inaccessible terrain.
"Fire is spotting ahead of the north eastern edge of the main fire front. Flames may be visible from southern Canberra suburbs. Firefighters are monitoring this activity. There is currently no threat to homes," the Emergency Services Agency said.
"Community members in Tharwa Village, Boboyan Road, Apollo Road, Top Naas Road and Nass Road need to remain vigilant.
"People in Banks, Gordon, Conder, Calwell and Theodore should continue to monitor conditions and stay up to date.
"The Bureau of Meteorology has predicted increasingly challenging fire weather for the coming weekend, driven by extremely high temperatures and low humidity."
Fire crews will work on Friday to reinforce containment lines and attack the fire's main body on the north-east and eastern side of the fire.
"Overnight we saw a number of spot fires ahead of the main body, and those spot fires have now been absorbed into the main body," Ms Whelan said.
The commissioner said those in southern Tuggeranong suburbs who intended to stay and defend their property should have a bushfire survival plan in place and know where to shelter should an emergency warning be issued.
"Our community needs to be clear on what their plan is and understand that plan and take appropriate action to protect themselves," Ms Whelan said.
"On our worst fire day, we won't be able to to put a firetruck on every corner and at every home.
"People need to be alert and not paralysed with fear by the environment around them by the consequences of not being prepared."
A NSW RFS issued fire a spread prediction on Thursday night, showing the fire moving closer to the small village of Tharwa on Friday.
The red areas of the map show the potential spread for the fire while the dotted areas are locations that may experience burning embers.
Friday is expected to see temperatures reach a top of 41 degrees with strong north-westerly and westerly winds forecast for the firefront.
Wind gusts of 40 to 50km/h are expected on Friday and Saturday, driven by a trough ahead of a front making its way across the ACT from western and central Australia.
Ms Whelan said she expected the fire to be "waking up" on Thursday, describing it as a "living being" that was behaving unpredictably.
Two strike teams from Queensland have arrived to help fight the blaze, and will stay in Canberra until Monday.
A community meeting will be held in Tharwa at 10am on Friday at the Tharwa Community Hall.
Authorities say it's the most serious blaze Canberra has faced since the deadly 2003 bushfires, which destroyed almost 500 homes and killed four people.
The territory's emergency services minister warned a state of emergency was possible to be declared with police ready for evacuations.
Canberrans were urged to stay out of the south if they didn't have a good reason to be there.
Three NSW rural firefighters were injured late on Wednesday after a tree collapsed onto their truck, with a fourth escaping uninjured.
Ms Whelan said all four had been hospitalised but were doing well.
Locals have also been warned against "disaster tourism", as authorities were forced to close roads to the public to avoid people travelling to capture images of the fire.
The Orroral Valley fire was spreading slowly in all directions, with the main firefront 10 kilometres wide and flames one metre high.
The fire was sparked by a defence helicopter's landing light after it touched down in Namadgi National Park while on reconnaissance.
Multiple road closures are in place due to the fires.
Roads closed include Tharwa Drive at the Knoke Avenue/Pockett Drive roundabout, Tidbinbilla Road from Point Hut Road, Corin Road from the Square Rock car park, Orroral Road and Apollo Road from the Namadgi National Park entrance, Angle Crossing at the Monaro Highway and Boboyan Road at the NSW/ACT border.