Most of the "megafauna" species had disappeared by the time people arrived on the continent, they said, challenging the claim humans were primarily responsible for the demise of the megafauna in a proposed "extinction window" between 40,000 and 50,000 years ago.
"The interpretation that humans drove the extinction rests on assumptions that increasingly have been shown to be incorrect," lead study author Stephen Wroe of the University of New South Wales said.
Scientists estimate 90 giant animal species once inhabited the ancient continent of Sahul, which included present-day mainland Australia, New Guinea and Tasmania.
"These leviathans included the largest marsupial that ever lived -- the rhinoceros-sized Diprotodon -- and short-faced kangaroos so big we can't even be sure they could hop," Wroe said in a university release Monday.
There is only firm evidence for about 8 to 14 megafauna species still existing when Aboriginal people arrived, the researcher report in their study.
"There has never been any direct evidence of humans preying on extinct megafauna in Sahul, or even of a tool-kit that was appropriate for big-game hunting," Wroe said.
Climate change is a much more likely suspect in the extinction of many megafauna species, he said.
"It is now increasingly clear that the disappearance of the megafauna of Sahul took place over tens, if not hundreds, of millennia under the influence of inexorable, albeit erratic, climatic deterioration."
The study by Australian and U.S. researchers has been published in the Proceedings of the National Academy of Sciences.