"It didn't do a great deal overnight, but crews remain on the scene," said RFS spokesman Bradley Collins.

"We are still concerned, but the Tianjara fire is where our focus is today, as it has the potential to impact the highway."

Western Road and a section of Comberton Grange Road have been closed.

UPDATE 8.31PM: The Comberton fire has been downgraded to advice, the RFS says.

It has burnt out 45ha, and is not yet under control.

All roads in the area are currently open.

UPDATE 7.42PM: South East Local Land Services has advised any land owners with large animals affected by the fire can take them to Nowra Showgrounds.

"People affected by the Forest Rd fire who have large animals that can be led on a lead or fitted with a halter and handled, can take them to the self care Large Animal evacuation centre at Nowra Showground - access via the Worrigee St entrance," they said.

"Animal owners are reminded that care of your animals including adequate food supply, as well as full time supervision remains your responsibility once you arrive at the evacuation centre.

"Owners are also asked to bring appropriate identification for their animals."

UPDATE 7.26PM: It is understood Forest Road has been reopened.

UPDATE 7.05PM: Forest Road has been closed at the Princes Highway in both directions.

RFS advice is as follows:

"If you are in the surrounding areas, monitor conditions. Know what you will do if the fire threatens.

"Check and follow your bush fire survival plan. Leaving early is your safest option.

"If you are threatened by fire, do not be in the path of the fire, protect yourself from the heat of the fire. Wear protective clothing and footwear. Cover all exposed skin.

"If the fire impacts, seek shelter in a solid structure to protect yourself from the heat of the fire.

"If your life is at risk, call Triple Zero (000).

"If you are in an area that has already been affected by fire, there may be small areas of active fire burning for some time. Be aware of the dangers of trees and branches falling."

UPDATE 6:45PM: Aircraft are dropping fire-retardant on the western edge of the fire.

A large air tanker has also been deployed.

UPDATE 6.30PM: LiveTraffic says traffic on the Princes Highway is being affected by smoke from the Comberton fire.

Motorists are advised to take care in the area.

All roads are still open. Shoalhaven City Council has adjourned their general meeting due to concerns about the fire.

UPDATED 5.46PM: The fire has been upgraded to Watch and Act on Fires Near Me.

It is burning three kilometres east of the highway and heading in a westerly direction, the RFS says.

"Crews are doorknocking homes around Comberton Grange Road to make sure they are aware of conditions," an RFS spokesperson said.

The fire broke out about 2.15pm and is about 10ha in size, the spokesperson said.

5PM: A fire near Forest Road and the Princes Highway at Comberton has the potential to close the highway, an RFS spokesperson has said.

The fire broke out after 3pm on Tuesday, December 17. It is not yet known how large the fire is, but ten RFS and Fire and Rescue trucks are already on scene, with five more from the Illawarra on their way, according to RFS spokesman Brad Collins.

"It is a going fire, and we have requested additional resources, including aircraft for mapping and water drops," he said.

"It is currently in a solid block off Forest Road and is inaccessible. We are looking at containment options."

These options include back burning. Mr Collins warned residents and those travelling on the highway to be prepared for heavy smoke, especially if backburning was undertaken.

"The highway may have to be closed, and if it is it will be between Forest Road and Jervis Bay Road," he said.