Gigantic animals which once roamed Australia were mostly extinct by the time humans arrived, according to a new study Tuesday which suggests climate change played the key role in their demise.

For decades, debate has centred on what wiped out megafauna such as the rhinoceros-sized, wombat-like Diprotodon, the largest known lizard, and kangaroos so big that scientists are studying whether they could hop.

The study, published in the Proceedings of the National Academy of Sciences, said some species were still surviving when people arrived about 45,000 to 50,000 years ago.

But the review, led by the University of New South Wales, found that while human involvement in the disappearance of the megafauna was possible, climate change was the more likely culprit.

"There is no firm evidence whatsoever that a single human ever killed a single individual megafauna," the study's lead author, University of New South Wales zoologist Stephen Wroe told AFP.

"Not a thing. There is not a single kill site in Australia or (Papua) New Guinea. There's not even the sort of tool kit that you would typically associate for hunter gatherers with killing big animals."

Wroe said the fossil records showed that the clear majority of now extinct species of megafauna "can't be placed within even 50,000 years of when humans were thought to have first arrived".

"No more than about 14, perhaps as few as eight, species were clearly here when humans made foot-fall," he said.

Wroe said there was also mounting evidence that their extinction took place over tens, if not hundreds, of millennia during which time there was a progressive deterioration in the climate.

"There is clear evidence that the climate was changing over a long period of time and becoming progressively more extreme," he said, adding this could have been harsh enough to kill off the giant animals, many of which were herbivores.

Some 90 giant animal species once inhabited Australia and Papua New Guinea -- including Diprotodons weighing close to three tonnes and kangaroos weighing up to 300 kilograms -- but their massive size did not ensure their survival.

"You think you've got these big hairy, often fierce beasts and they'll be able to look after themselves, but the cruel irony is that the biggest and fiercest... can be extremely vulnerable," Wroe said.