The Currowan fire on the South Coast has broken containment lines and forced the closure of the Kings Highway again.
The fire, which has been burning north of Batemans Bay for nearly three weeks, jumped Currowan Creek before 4pm on Sunday.
An active fire front of about one kilometre was approaching the Kings Highway near Misty Mountain Road and Government Bend at about 5pm on Sunday. As of 8pm, the fire was as little as 30 metres away from the highway in sections.
Waterbombing aircraft was sent in to contain the spread.
The highway remains closed to traffic between Braidwood and Nelligen as of 8am on Monday and it is not known when the road will reopen to drivers.
The road only reopened late on Friday, after being shut for more than a week due to firefighting and backburning operations.
With Araluen Road also closed, any Canberrans trying to get from the South Coast back home could be in for a difficult time.
However, fire crews have said they intend to reopen the Kings Highway as soon as possible, but warned of more backburning in the area within the next 24 hours.
Alan Henderson from the Shoalhaven Rural Fire Service said the road posed dangers to drivers.
"There's also the risk of tree fall in the area and we've got to make sure we've assessed and treated the area," he said.
Motorists were being told to use the Princes Highway, Albatross Road, Braidwood Road and Nerriga Road. People should use the Livetraffic website to monitor the road closures.
Fire crews said conditions were favourable overnight for firefighters working to contain the blaze, as the fire moved north.
"Everything is pretty much back on track to continue the containment," Mr Henderson said.
"The fire has moved further north, so we'll be looking to containment strategies to manage that, and that's going to happen quicker than expected.
"On Monday, we've got a southerly and easterly influence, which is pretty positive."
NSW Rural Fire Service officer Marty Webster said the fire had been moving quickly uphill when it jumped containment lines on Sunday, and firefighters were actively attacking the blaze.
It was in an awkward spot and firefighters were disappointed the fire had crossed the creek, Mr Webster said.
"We were hoping to hold it along the creek but that's no longer possible and we've had to fall back to the Kings Highway," he said.
However there is currently no threat to homes or assets.
Mr Webster said there was also active fire at the other end of the Currowan fire, near Pigeon House Mountain.
Residents in the Milton area would likely see increased smoke and fire would likely be visible once it gets dark. However, there was no threat to properties.
The fire has burnt out nearly 90,000 hectares since it began on November 26.
The fire is at an advice level, as of 8am Monday.
Mr Henderson said crews were bracing for hot conditions later in the week, as heatwave conditions head east.
Temperatures near Canberra are expected to reach into the 40s later in the week.
"The coast isn't going to be as hot, but it's certainly going to be warming up."
There were more than 250 volunteer firefighters out on the fireground on Sunday.
Dry and gusty conditions fuelled an increase in fire activity on both the northern and southern edge of the fireground.
"The air's been quite dry again, with low humidity and gusty winds [which has been driving the fire]," Mr Webster said.