Investigators say Gordon Rogers was last seen at a party in the town of Beachport, about 400 kilometres southeast of Adelaide, in the early morning hours of Aug. 2, 1970. He was 20 at the time.
Police say they suspect he was murdered.
Now a $200,000 reward is being offered to anyone who provides information that leads to his remains or an arrest in the case.
Rogers' disappearance is among more than 100 cold cases featured in a campaign that South Australian police say has generated hundreds of tips since February.
They say Rogers arrived in Sydney on a 12-month visa in March 1970 and travelled a bit before taking a labourer job with the Woods and Forest Department.
Police say he planned to work with the department for six weeks before travelling to New Zealand.
The night he disappeared, Rogers was at a party with a friend who then left to take his girlfriend home, police said. The friend asked another man to take Rogers home.
That man later told authorities he took a woman home and when he returned to the party, Rogers was gone.
Police say no one has seen or heard from him since.