Beaumont children disappearance:
https://en.wikipedia.org/wiki/Beaumo..._disappearance
I'm not from Australia, but this is apparently one of the country's most famous cold cases, dating from 1966. Three children are playing at the beach, and eye-witnesses suggest that they befriended a tall blonde man. They disappear and haven't been heard from since. No bodies, no evidence, and no firm leads.
There's a lot of interesting theories and proxy stories about other abductions and murders. There are 4 main suspects, including "The Family," a group of men believed to be involved in the murder of 5 boys.