The 500million figure would seem to stem from a University of NSW figure from Prof. Chris Dickman and was for animals "affected" rather than those necessarily killed, although the immediate prospects (food, water, shelter, degree of injury,etc..) of all of the affected animals would likely be bleak.
The figure includes mammals (~18 per hectare), birds (~21 per hectare) and reptiles (~130 per hectare) with the densities based on a 2007 study/paper and also based on when the fire area was 3 Million hectares (now 5 Million hectares). If extrapolated to the current 5 million hectare bushfire area the figure would be over 800 million animals.
Here is a related link....
https://www.bbc.com/news/50986293
Best
JA