Paul Elliot said he and his son Jyah found the bottle on the west coast of Eyre Peninsula during a fishing trip.
The fisherman told the Australian Broadcasting Corporation he is searching for the note’s author, who would now be 63-years-old.
The note describes the journey of Paul Gibson, a 13-year-old English boy travelling on a cruise ship along the southern Australian coast from Fremantle in the west to Melbourne in the east.
David Griffin, a government oceanographer, said the bottle could not have remained afloat for 50 years off the south coast because "the ocean never stays still".
“If it had been dropped in anywhere in the ocean somewhere south of Australia, then there’s no way it’s going to stay actually at sea moving around for more than a year or two,” he said.
Hundreds of thousands of Britons migrated to Australia in the 1960s with the Australian government subsidising their fares. Children travelled for free.
But a quarter of them returned to Britain within a few years when life in Australia fell short of their expectations.
The world’s oldest known message in a bottle was discovered by a couple on a beach in western Australia last year. The bottle is thought to be around 132 years old and contained a note written in German dated 12 June 1886.
Additional reporting by AP