The discovery of a giant platypus at the World Heritage-listed Riversleigh site in Queensland's north-west has changed the way scientists believe the animal evolved.

The giant platypus is thought to have been a metre long, with large teeth used to kill and eat animals like turtles, and is between five and 15 million years old.

Professor Mike Archer was part of the group that discovered the fossil and says previously it believed that only one type of platypus existed in Australia at the same time.

"Now we have to be on our toes, we thought there was only one," he said.

"In all other fossil deposits in Australia where there are fossil platypuses, there's always only one species at a time.

"All of a sudden Riversleigh has given us two - so I guess we're going to be much more intently keeping an eye out for the possibility of maybe it was even more complicated than that."

He says what was found has surprised scientists in "a whole range of different ways".

"It's a gigantic platypus, twice as big as the biggest platypus today ever dreamed of becoming," he said.

"This is a platypus on steroids and it's defying our understanding of what's been happening to platypuses over time."