A Belgian man says an encrypted phone message sent by his missing son could hold a clue to the 18-year-old tourist's disappearance from an Australian coastal town.

Theo Hayez was last seen leaving a Byron Bay nightclub late on May 31. Police said on Monday they are baffled by the disappearance.

His father Laurent Hayez flew from Belgium to Australia last week to take part in the search which now involves homicide detectives.

Hayez on Monday made a public appeal for help to access his son's encrypted WhatsApp account.

Police Superintendent Dave Roptell said WhatsApp's owner Facebook was being asked for access to the missing man's account.