Its teeth shows it only ate plants and its arms suggest it would scratch for food on the ground, such as when looking for roots.
Diprotodon has that honour, weighing in at an impressive 2,000kg - two tonnes - and surviving until at least 50,000 years ago.
"But animals like mukupirna show that their extinct relatives were even more extraordinary, and many of them were giants."
Archaeologists used an "acupuncture" method to find the bones, pushing metal rods into the soft mud until they hit something hard before digging it up.
Mukupirna is now the closest known relative of modern wombats, yet it is still so different scientists have given it its own family of creatures - mukupirnidae.
Professor Michael Archer of the University of New South Wales, who was part of the team that first uncovered the bones which have only just been identified in 1975, hailed it as a "mysterious new beast".
Julien Louys of Griffith University, who co-authored the study, said: "The description of this new family fills a crucial missing piece to the ancient bestiary of Australia.