A COIN found in a swamp could help prove a Spanish or Portuguese ship was wrecked on Australia's east coast years before Captain James Cook's voyage.

The coin, found in a snake-infested marsh, could help prove a century-old theory that a Spanish or Portuguese ship was wrecked on Australia’s east coast years before Captain Cook’s famed voyage of discovery.

The find, made by an expedition led by self-funded Brisbane historian Greg Jeffreys, is the first piece of dated evidence among a number of artefacts found in Eighteen Mile Swamp on Queensland’s North Stradbroke Island.