SYDNEY (BLOOMBERG) - As many as a billion animals may have been killed in Australia's wildfires since September, a scientist said, doubling his earlier estimate as the unprecedented scale of the crisis in the world's driest inhabited continent continued to emerge.

New figures released on Wednesday (Jan 8) by the University of Sydney's Chris Dickman indicate more than 800 million animals have been killed in the state of New South Wales alone, while one billion had died nationally. That includes mammals, birds and reptiles directly killed by the fires or indirectly through loss of habitat.

"We're probably looking at what climate change may look like for other parts of the world in the first stages in Australia at the moment," said Prof Dickman, a professor in ecology in the school of environmental science, said in a statement on the university's website, noting that events like this may hasten the extinction process for a range of species. "It's a very sad time."

Distressing images of injured or dead Australian native animals - including koalas and kangaroos - have been flooding social media streams as the wildfires sweep through south-eastern Australia, destroying vast tracts of land and homes. The human death toll stands at 25.

More than 10 million hectares have been destroyed so far - that's larger than the US state of Indiana - while smoke from the fires has spread halfway across the globe, darkening skies in Argentina and into the Atlantic. The fires are so large they're generating their own weather systems and causing dry lightning strikes that in turn ignite more blazes.

Koalas have been particularly affected, according to Environment Minister Sussan Ley, who told local radio last Saturday that up to 30 per cent of the population on the mid-north coast of New South Wales may have been killed.

Prof Dickman said earlier: "With the type of fast-moving crown fires that we have been experiencing, koalas really have no capacity to move fast enough to get away.

"There is such a big area now that is still on fire and still burning that we will probably never find the bodies."