Crews are working to contain the North Black Range fire west of Braidwood on Sunday as the conditions are set to deteriorate on Monday and Tuesday.
At 11am Sunday, the bushfire was burning at "advice" level and had burnt more than 35,000 hectares of land.
Fire activity was easing across the fireground but remained active in Warri, Bombay, Mount Elrington, Farringdon, Harolds Cross, Forbes Creek and Butmaroo.
NSW Rural Fire Service spokesman Daniel Osborne said "it was a big day" for firefighters who were taking advantage of favourable conditions to conduct containment and backburning operations.
He said overnight crews worked on backburning in Harolds Cross north of Vernelly Road and in the Butmaroo area. Crews will continue to patrol those areas on Sunday.
"There may be further backburning in areas around the fireground today as conditions allow and as we really take advantage of that more favourable weather we have today and tonight," said Mr Osborne.
Mr Osborne said firefighters would be targeting their efforts on the entire fireground on Sunday but the north eastern part of the fire was a focus.
"We've got a lot of focus today on the north, north eastern part of the fire in the Warri area and we're working to identify and constructed containment lines in the Jones Road, Watsons Road and Manor Road area," he said.
Heatwave conditions are forecast in the Braidwood region on Monday and Tuesday, with a top of 35 degrees and 38 degrees, respectively.
Temperatures could reach up to 40 degrees near the fireground on Tuesday, according to the Bureau of Meteorology.
On Sunday, the Kings Highway was closed from Braidwood to Nelligen due to back burning operations for the Currowan fire. The Kings Highway north of Braidwood is open at a reduced speed limit fue to heavy smoke.
Araluen Road was also closed between Braidwood and Moruya, but residents may be permitted access at the discretion of emergency services.
Tallaganda National Park, State Conservation Area and State Forest remain closed.