As an environmental scientist (in a former life) and based in Australia, I will say that other threats such as land clearing, attacks from dogs, chlamydia and other human-influenced factors have been adversely affecting koala populations across the country more than bacterial infections.

I wonder to what extent the species' population decline (thus, less genetic diversity) over time has contributed to its increased tendency to be susceptible to bacterial infections.

Strictly speaking, koalas (Phascolarctos cinereus) are only present along the east coast, as they're not found across the entire continent, despite the land mass.

P.S. Contrary to popular belief, 🐨🐨 are not bears. Rather, they're arboreal (tree-dwelling) marsupials.

https://www.savethekoala.com/our-work/bobs-map-koala-populations-then-and-now/