The North Black Range bushfire has been upgraded to emergency and some residents south-west of Braidwood have been told it is too late to leave.
The fire was upgraded from advice level to "emergency" about 4pm on Thursday.
It is too late for residents in the areas of Mount Elrington and Wallaces Gap to leave and they have been advised to seek shelter and protect themselves from the heat of the fire.
NSW Rural Fire Service officer Darren Marks said there had been a significant increase in the fire spotting to the north and south, with spotting occurring across Cooma Road.
He said with any luck crews would be able to contain the fire to the north of populated areas.
"We're flooding the area with resources and with any luck we'll be able to hold them up," said Mr Marks.
Mr Marks said the worst case scenario would be that the fire would reach the Monga National Park.
The fire forced the closure of the Kings Highway about 1.45pm on Thursday between Columbo Road and Mayfield Road.
Cooma Road is also closed south of Braidwood.
Firefighters may experience a reprieve from the disastrous conditions on Thursday evening, but Mr Marks said that would not be before 7pm.
There will be another total fire ban in the Braidwood region on Friday.
It has burnt more than 27,000 hectares near the Tallaganda National Park.
Westerly and north-westerly winds of up to 50km/h have been felt across the fireground, impacting the northern and southern parts of the bushfire.
The fire is also moving in a northerly direction near the Shoalhaven River and Sheep Station Creek.
Containment areas near Colombo Road are also threatened due to the strong winds.
Residents are being urged to activate their bushfire survival plan as conditions inteisify. Picture: Dion Georgopoulos
Spot fires have been seen around the south of the fire near Farringdon.
Residents in the area who weren't intending to stay and defend their properties were urged to leave before 8am on Thursday.
People who have been displaced by the fire have been urged to go to the National Theatre on Wallace Street in Braidwood to wait out the conditions.
Queanbeyan fire patrol centre spokesman Daniel Osborne said challenging conditions for firefighters were ahead as the intensity of the fire increased on Thursday morning.
"We're throwing everything we can at this fire, because we know the risks," Mr Osborne said.
"The fire rating across the area is at severe today, which means trying to contain the fires and extinguish them is going to be extremely challenging.
"Our firefighters will do their best to do what they can and protect what they can but containing the fire today is highly unlikely."
The fire remains burning to the west of Cooma Road, but Mr Osborne said drivers should be prepared for roads in the area to close at short notice.
An estimated 180 firefighters are battling the fire, along with crews from the ACT and South Australia.
Aircraft are also being used to help control the fire.
Schools in Braidwood have closed on Thursday, including Braidwood Central School, Braidwood Preschool and St Bede's Primary School.