Around 50 firefighters are battling a bushfire burning out of control near properties in Sydney's north.

NSW Rural Fire Service (RFS) said the 200-hectare fire was burning at Lovett Bay in the Ku-ring-gai national park.

An RFS spokesman told AAP shortly before 10.30am (AEDT) on Saturday the fire was burning near properties but was not a direct threat to any of them.

He said there were no mandatory evacuations being carried out but campers at The Basin camping ground in the national park had been advised to leave.

They were being transported out by ferry because West Head Road was closed, he said.

The cause of the fire was unknown, the RFS said.

Motorists were being advised to avoid the area.

On its website, the RFS said firefighters were working to establish containment lines.

Smoke was impacting Terrey Hills, the northern beaches and north shore.