SYDNEY (BLOOMBERG) - Close to half a billion animals in Australia's New South Wales state may have been killed in wildfires since September - and the number could climb.

Research, led by the University of Sydney's Chris Dickman, said many of the estimated 480 million affected mammals, birds and reptiles would have been directly killed by the fires or indirectly through loss of habitat.

"Prior research indicates that in severely burned areas, the resulting lack of shelter, lack of food, and incursions by invasive predators - red foxes and feral cats - lead to further drastic but indirect reductions of animal numbers," Professor Dickman said Friday (Jan 3).

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 since the fire season began months ago in the southern hemisphere winter stands at 20.

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

Nature Conservation Council ecologist Mark Graham also highlighted the plight of koalas last month when he spoke at a state parliamentary inquiry into the diminishing population.

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

Even still, the 480 million figure really only provides a rudimentary indication of the true scale of ecological destruction, according to Prof Dickman, who highlighted it was a conservative estimate.

The fires have destroyed more than 12 million acres (5 million hectares) - an area that's more than twice the size of Wales.