A Sydney beachgoer has undergone emergency surgery after being struck in the back of the head near a rock pool.
The 26-year-old man was assaulted at Fairlight, near Manly, on Sunday afternoon after he and his friends were in an argument with a group of four men, police said on Monday.
The man was struck from behind, causing him to fall with his head striking the ground, detectives were told.
He underwent surgery after his condition deteriorated at Royal North Shore Hospital, where he remains in a critical condition.
Witnesses are urged to contact local police or Crime Stoppers.