Mark Anthony Barnes, aged 23 years, was last seen on 10 November 1986 while working for the Australian National Railways at Cook, on the Nullarbor Plains in South Australia.
He has not had any contact with his friends, family or any government or medical agencies since his disappearance.
Anyone with information which may assist in locating the whereabouts of Mark are urged to anonymously contact Crime Stoppers on their website or free call 1800 333 000.