The man was seen in distress in the water at Main Beach at Lennox Head, south of Byron Bay, about 3.20pm yesterday.
Police said it was reported he “failed to resurface”.
Local police, surf lifesavers, Marine Rescue NSW and a rescue helicopter were searching for the man overnight.
The man has been described as Caucasian with brown hair.
Police are urging anyone with information to contact Ballina police or Crime Stoppers on 1800 333 000.