A bushfire west of Jervis Bay has burned through 223 hectares of national park land in less than 48 hours.
Firefighters have contained the blaze at Jerrawangala National Park, which was first noticed on Monday morning.
Upon arrival, firefighters discovered 40 hectares of bushland had been burning overnight in an inaccessible area.
National Parks and NSW Rural Fire Service crews launched backburning operations to prevent its spread.
"We realised there was no possible way we could extinguish it without undertaking backburning," RFS Shoalhaven spokesman Brad Collins said.
"The crews on ground did well to run an edge around it, they covered a fair distance."
RFS crews from Tomerong, St George's Basin, Basin View and Wreck Bay, including the Shoalhaven duty officer were on the job, with National Parks teams.
On Tuesday, National Parks crews remained on scene to patrol the fire, which continues to burn.
"People driving along Turpentine Road may notice there's an active fire along the roadway," Mr Collins said.
"It's no cause for alarm."
While the cause of the fire is undetermined, the RFS has been treating it as suspicious.
"There has been no lightning over the previous days," Mr Collins said.