Firefighters are predicting a 16,000 hectare blaze south of Canberra to creep closer to the city as temperatures spike and winds increase.
The NSW RFS issued a fire spread prediction on Thursday night, showing the fire moving closer to the small village of Tharwa on Friday.
ACT Emergency Services Commissioner Georgeina Whelan said hot and windy conditions plus a potential thunderstorm are expected over the weekend.
The fire, which has burned through 16,571 ha, is nine kilometres from the capital's southern outskirts and three kilometres from the small village of Tharwa.
"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," it 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."
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 fire front 10km wide and flames one metre high.