An autistic man has spent a night in the bush after disappearing in Sydney's Ku-Ring-Gai National Park.

The 40-year-old was last seen about 5pm (AEST) on Saturday at a marina in Akuna Bay but was found safe and well on Sunday morning.

The search resumed on Sunday morning with local police, the Police air-wing, PolAir, the SES and Volunteer Marine Rescue.

The man was found on the road about 8.45am on Sunday not far from where he was last seen.

Police believe he spent the night in the open.

"Unfortunately due to the circumstances of his condition he has been unable to tell police where he has been overnight but the good news is he does appear to be safe and pretty well, despite being outside overnight," a police spokeswoman said.