A coin collector has revealed an unassuming coin that could be hiding in anybody's spare change could be worth more than 80 times its value to prospective buyers.

The Perth Mint and the Australian Post Office have released a set of limited edition coins and stamps commemorating World Youth Day 2008.

Australia's central bank says its new $5 bill would feature an Indigenous design rather than an image of King Charles III. But the king is still expected to appear on coins.