Claims that over a billion animals perished in NSW fires seems to have been pulled out of thin air.

I first heard the claim in early January - while the fires were still raging - that half a billion animals had perished in the Australian bushfires. This figure quickly and inexplicably doubled just over a week later to a billion.
Now, ABC News in a dramatic claim is reporting that "more than a billion animals are thought to have perished in NSW alone" 0:35.
How on earth did they come up with these sensational figures so quickly? Aren't these the same people who are always demanding peer-reviewed evidence? Almost certainly, there is no reliable data to back -up this claim, let alone anything that's peer-reviewed - which has always been their standard in the past - or when it suits.
Airdate: 18-02-20