A total of 800 million animals are now thought to have died in New South Wales bushfires.

The figure is almost double initial estimates made by University of Sydney academics, who said last week 480 million animals had been killed in the fires.

A koala caught in bushfire flames on the NSW Central Coast. (9News)

The authors of the confronting revelations used World Wild Fund estimates for mammal density in NSW, and calculated how it would extrapolate to bushfire-destroyed areas.

Professor Dickman, from the Faculty of Science, said: "I think there's nothing quite to compare with the devastation that's going on over such a large area so quickly.

"It's a monstrous event in terms of geography and the number of individual animals affected."

The animals estimated to be killed would have died from either the fires themselves, or the depletion of food and shelter and predation from feral cats and foxes.

Steve Shipton prepares to shoot an injured calf in his paddock after a bushfire in Coolagolite, NSW, January 1, 2020. (AAP)

Some will have gone underground to shelter, or flee, and end up in areas without the resources needed to survive.

That figure does not include insects, bats or frogs, and only includes animals killed in NSW.

The death toll for animals killed in bushfires in Victoria, Queensland, Western Australia, South Australia and Tasmania is not known.

Professor Dickman, said it is an effect of climate change, and warned it could lead to extinctions.

The remains of a car yard in the industrial estate at Batemans Bay. (Kate Geraghty)

Thirty-four mammal species in Australia have become extinct since European colonisation, the highest rate of loss for any region on Earth.

There are more than 300 native mammal species on the continent.

"We know that Australian biodiversity has been going down over the last several decades, and it's probably fairly well known that Australia's got the world's highest rate of extinction for mammals," he said.

"It's events like this that may well hasten the extinction process for a range of other species. So, it's a very sad time."