Multiple crews of Rural Fire Service firefighters and aircraft are responding to the bushfires, burning in the park along Sir Bertram Stevens Drive at Flat Rock and south of Wattamolla Road.

The park is closed at McKell Avenue and Farnell Avenue and visitors are being advised to avoid the area.

Fire crews are unsure how many people are in the park, which is a popular weekend destination, but have warned visitors to leave "only if the path is clear" or if directed by emergency services.

The RFS has issued an emergency warning and is sending telephone warnings to people in the area.

Visitors have been advised to head to the beaches, which "may offer safety".

NSW RFS Deputy Commissioner Rob Rogers told Nine News there were 40 fires burning throughout the state on Saturday afternoon, and 18 of those were out of control.

He said the RFS were "very concerned" about the cause of the Royal National Park fires, which is not yet known.

"We do have fire investigators currently working with local police, investigating the cause and origin of this fire," he said.

Marnie Sigal, who owns a kayaking company based in Bundeena, said she drove along Sir Bertrams Stevens Drive towards Bundeena at about 12.35pm on Saturday and didn't spot the bushfire.

Ms Sigal was driving back from Audley, transporting 14 people who had been kayaking with her company.

Just 10 minutes later her family members passing the same spot drove past flames right near the road.

"I'm talking five minutes, maybe 10 minutes at the most," she said of the distance between her vehicle and the two cars carrying her son and father, and her husband and the kayak tour leader.

"To go from nothing to quite a big, decent, out of control fire - I couldn't believe it.

"My mum and dad's car is covered in ash."

Ms Sigal said her husband, who is a member of Fire & Rescue NSW, and the tour leader, who is in the RFS, both reported the fire.

She said she had been "surprised" to receive a text alert about the nearby bushfire "for the first time ever".

A spokesman for NSW RFS said around 70,000 messages were sent to people in the Bundeena and Wattamolla areas "where hundreds of residents were expected to be enjoying the area's beaches".

"The effectiveness of the warning would have been hampered by the lack of mobile phone reception in the national park," he 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."

Those already at Little Garie, Garie, North Era, South Era or Burning Palms beaches have been told to remain in place.

On Saturday afternoon, an RFS spokesman said there was no direct threat to properties, but residents in the Bundeena area are being warned to enact their bushfire survival plans.

The fire is emitting large amounts of smoke, which can be seen right across Sydney.

Parts of the park at Wattamolla had been closed earlier on Saturday as the car parks were "full".

NSW RFS and NSW Police began evacuating Wattamolla Road in the Royal National Park on Saturday afternoon.

"We are escorting around 200 people out of Wattamolla Road, this is undertaken in conjunction with NSW Police to get some of those people out, and will continue throughout the afternoon," the RFS spokesman said.

Marine Rescue NSW has sent vessels from Sydney and Illawarra to Burning Palms Beach in Lilyvale, "due to escalation of a bushfire in that area and possible water-based evacuations".

Burning Palms Beach is just north of the Figure 8 Pools, a popular tourist destination in the Royal National Park.

It is understood a surfing carnival is taking place within the park, but the RFS do not believe there is any threat to visitors at the event at this stage.

Meanwhile, the RFS has issued a "watch and act" alert for a bushfire burning out of control south-west of Sydney at Long Gully Road in Bannaby.

While firefighters and aircraft are working to slow the fire, the weather conditions are causing fires to "burn erratically" and become "difficult to control".

Isolated rural properties in the area of Alders and Crees Road and Bannaby Road may soon come under threat, the RFS warns.

Firefighters are also continuing to battle with a bushfire that's burnt more than 33,000 hectares in the Pilliga Forest between Coonabarabran and Narrabri.

The fire is at an advice level with no homes are under threat, but residents are being urged to monitor conditions.

Firefighters are currently conducting back burning operations in the area and the Newell Highway has been shut between Narrabri and Coonabarabran and is likely to remain closed on Saturday.

Further south, firefighters from the ACT have been sent across the NSW border to protect houses threatened by a bushfire in the Southern Tablelands.

Three heavy tankers, backed up by smaller crews, were deployed to the 274 hectare Braidwood blaze on Saturday morning after it burned through the night.

"Their objectives are to protect properties, attack and contain the main fire and contain spot fires," the ACT Emergency Services Agency said.

with AAP