The blaze, which appears to have been deliberately lit on Saturday, has burnt through more than 600 hectares.
Firefighters worked through the night to contain it and will continue carrying out back burning on the eastern side on Sunday, the RFS said.
The national park will remain closed on Sunday and Monday with access only for residents.
The blaze is being treated as suspicious with specialist investigators heading into the park on Sunday, a NSW Police spokeswoman told AAP.
RFS Deputy Commissioner Rob Rogers said it was lucky nobody was killed in the blaze and it's infuriating people could think of starting a fire on purpose.
"It is absolutely frustrating as I think the average person in the community finds its just unbelievable that people would do that. Given there was more than one fire obviously we are concerned about how the fires started," he told Network Seven.
The frustrating thing is that luckily there was no one killed, but it could have quite easily been very different if it wasn't for all the agencies working together. And look at the cost - this firefighting effort will cost millions of dollars. There is a cost of these things, let alone that trauma to the individuals," Mr Rogers said.
Hundreds of visitors to the Royal National Park were rescued by boat from beaches after becoming stranded, according to the RFS.
"There are a number of boats being utilised by surf lifesavers, NSW Police Force and some private citizens that are picking some people up from beaches in the area," RFS spokesman James Morris told AAP during the drama.
More than 100 firefighters battled the blaze on Saturday evening, supported by water bombers and aircraft.
A large blaze north of Tamworth, in the state's north, threatened properties close to Bundarra while firefighters brought a fire under control near Taralga that had also endangered homes.
Both blazes have been downgraded to Watch and Act, the RFS said Sunday.
NSW has endured hot, dry temperatures for several days, with Sunday expected to bring no relief.
Total fire bans have been declared for the Southern Ranges, Central Ranges, Greater Hunter and North Western regions on Sunday.
