Update Monday
Police are treating the Royal National Park fire as “suspicious” and a senior Rural Fire Service (RFS) officer said it was lucky no one was killed.
RFS and police investigators were reported to have found multiple ignition points for the fire, which caused mass evacuation on Saturday.
Hundreds of visitors to the park were rescued by boat from beaches after becoming stranded.
Vessels from Marine Rescue NSW evacuated up to 80 people from Garie beach..
The national park will remain closed on Monday with access only for residents.
Two fires have burnt through more than 1800 hectares of the park.
Firefighters worked through the night to extinguish one of those blazes and were attempting to bring the second fire under control before worsening conditions forecast for Monday.
The blaze is being treated as suspicious, a NSW Police spokeswoman told AAP.
RFS Deputy Commissioner Rob Rogers said it was lucky nobody was killed in the blaze and it was infuriating that someone could think of starting a fire on purpose.
"It is absolutely frustrating as I think the average person in the community finds it's 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 the Seven Network.
Mr Rogers said it was lucky no one killed, but it could easily have been a very different story 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," he said.
Update:
The Rural Fire Service (RFS) says the fire continues to burn in the vicinity of McKell Avenue and Sir Bertram Stevens Drive.
RFS has provided vision taken from a RFS helicopter as it inspects damage to the park.
Update
Police believe the Royal National Park fire, which caused the evacuation of thousands of visitors on Saturday, a number of media outlets have reported.
9 News said firefighters had found multiple ignition points.
Meanwhile the National Park remained closed to all but residents on Sunday afternoon.
Update
Rural Fire Service says Royal National Park visitors who left vehicles at Bundeena yesterday can return using the Cronulla ferry service and drive out of the park
Update
Royal National Park remained closed on Sunday morning after the bushfire emergency on Saturday.
Rural Fire Service said access was available only only for residents.
“Crews have worked through the night to slow the fire which has burnt 600 plus hectares,” RFS said.
Earlier
The Royal National Park was closed and hundreds of people escorted to safety on Saturday, as firefighters battled two out-of control bushfires near Wattamolla in Sydney's south.
Two hundred people were evacuated from Wattamolla Road to Bundeena by NSW Rural Fire Service and NSW Police in the afternoon, while other visitors were advised to "head to the beaches".
"They're providing the best protection given the fact a lot of the escape routes are through the middle of the bushland which isn't safe at the moment given the intensity of the fire," a spokesman for NSW RFS said on Saturday.
Private boat owners alongside NSW Surf Life Saving, NSW Police and Marine Rescue NSW vessels at Garie Beach had evacuated up to 80 people by the evening.
"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," the spokesman said.
More than 100 firefighters were on the ground working to control the fires burning along Sir Bertram Stevens Drive at Flat Rock and south of Wattamolla Road. By evening 236 hectares of land had been burnt through.
People in Bundeena, Little Garie, Garie, North Era, South Era or Burning Palms were being advised to remain in place until directed to move by emergency services.
Poor mobile reception in the park was also an issue, with the RFS sending out about 70,000 text message alerts to people in the area of Bundeena and Wattamolla.
"The effectiveness of the warning would have been hampered by the lack of mobile phone reception in the national park," the RFS spokesman said.
"We would love to just be able to send push notifications out to people whether they have reception or not, but the technology doesn't make that possible yet."
The spokesman said a large number of people were still being observed on walking tracks by overhead planes on Saturday, despite the emergency.
"There are still a significant number of people in there," he said.
Marnie Sigal said she drove along Sir Bertram Road near Flat Rock just minutes before bushland next to the road was engulfed in flames.
Her husband and an employee of the kayaking business she owns, both of whom were off-duty firefighters, reported the fire as they travelled towards Bundeena just 10 minutes behind Ms Sigal's vehicle.
"I'm talking five minutes, maybe 10 at the most. To go from nothing to quite a big, decent, out of control fire - I couldn't believe it," she said.
All roads into the Royal National Park were closed and motorists advised to avoid the area, while people in south-western Sydney and the Illawarra were advised to be wary of significant amounts of smoke drifting over the regions from the fire.
The RFS had set the warning level to "watch and act" on Saturday evening.
RFS Deputy Commissioner Rob Rogers old Nine News the bushfire was being treated as suspicious. He said the RFS was "very concerned" about the cause of the fire, which was not yet known.
"We do have fire investigators currently working with local police, investigating the cause and origin of this fire," he said.
On Saturday evening there were 40 fires burning throughout NSW, 18 of which were out of control, he said.
The RFS said rural properties in the Alders and Crees Road areas in Bannaby, near Bowral, might come under threat from a grassfire.
The 634-hectare blaze was at watch-and-act alert level, with firefighters and an aircraft on scene to try to slow its spread.
A watch-and-act alert was also issued north of Tamworth, where there was an out-of-control 3500-hectare fire. The RFS was concerned isolated rural properties would be threatened 10 kilometres south-west of Bundarra.
Firefighters are conducting backburning operations in the area and the Newell Highway was shut between Narrabri and Coonabarabran on Saturday.
with AAP
