An extinct species of giant, toothed platypus, about a metre in length, has been discovered and identified in far north-west Queensland, using a single tooth.

The lower molar, originally unearthed in the Riversleigh world heritage fossil area near Mount Isa, could have stayed hidden in a cupboard at the University of NSW, were it not for palaeontology student Rebecca Pian.

"At first I didn't think it was anything; no one had really noticed it," said Ms Pian, who studied the creature for her honours research and who is completing a PhD at the Columbia University in New York.

"It was only when I started studying it that I realised it was really different from the others.