Police are calling for witnesses after a man was critically injured while cycling on the New South Wales central coast yesterday.

The 46-year-old victim was found lying on Barrenjoey Road at Ettalong Beach just after 2:30pm (AEST).

Police say he had been riding a bicycle, and they believe he was assaulted in a nearby street about 90 minutes before being found.

The man has a mental impairment.

He was taken to Gosford Hospital with head injuries, and then transferred to Sydney's Royal North Shore Hospital.

Police say anyone with information should contact Gosford Police or Crime Stoppers on 1800 333 000.