An increase in wind on Monday afternoon has produced a significant amount of smoke, mainly over parts of south-west Sydney, the Illawarra and the South Coast, an RFS spokesman said.
Residents are being urged to call triple zero only if they see a fire burning unattended.
Fire crews are patrolling containment lines and continuing to strengthen those lines, and there are no current threats to properties, the RFS said.
The fire, which burnt out of control on Saturday and Sunday, is being treated as suspicious.
The Royal National Park remains closed except for residents, and motorists have been advised to avoid the area.
The RFS is "hopeful" that the fire will be contained as soon as possible, and the national park will be able to reopen for the Australia Day long weekend.
The fire was downgraded to advice on Sunday, but conditions were expected to worsen on Monday with hot and dry conditions elevating the fire risk across many regions in the state.
The Bureau of Meteorology issued a fire weather warning on Monday, forecasting severe fire danger for the Greater Hunter, Greater Sydney, Southern Ranges and Central Ranges regions. A total fire ban is in place in those four regions.
At 9am more than 700 firefighters and 246 appliances are working on 35 fires across NSW, 13 which are uncontained. All currently at Advice. Our thanks to the #NSWRFS members who worked overnight to strengthen containment lines ahead of forecast hot conditions today. #nswfirespic.twitter.com/BoZXjFXqbi??? NSW RFS (@NSWRFS) January 21, 2018
Temperatures across greater Sydney are expected to remain high on Monday and Tuesday, with tops of 33 degrees and wind forecast in the Royal National Park and Sydney city; 43 degrees for Penrith and 39 for Parramatta.
The RFS said more than 600 firefighters were battling 35 bush and grass fires across NSW on Monday, including the Royal National Park blaze. Fourteen of the fires remain uncontained.
Hundreds of park visitors were rescued by boat after being advised to "head to the beaches" on Saturday.
The RFS said people who needed to collect their cars from Bundeena were able to pick them up on Sunday.
The story Firefighters continue to battle Royal National Park bushfire first appeared on The Sydney Morning Herald.