So... the characters in this series. They all speak English, and with an Australian accent, and with Australian slang. That would suggest that their version of Australia has a similar history to ours, right? Colonized by Brits, first with convicts, then with... not arrested people, right? So that would imply that British people (in this case, European animals) and their descendants would be around. But pretty much every animal seen in the games, with a few exceptions, is native to either Australia or its satellite nations. Which would imply that they're the equivalent of the Australian Aboriginals, right? But European animals (barring sheep) are pretty much never seen! So... did this version of Australia get reclaimed by its native people (well, animals), but cling to the colonial speech and mannerisms? Are they still there, just ofscreen, in a weird Funny Animal variant of Monochrome Casting? Or am I overthinking this?
You are definitely overthinking this. The simple reason the game features only native animals is because sheep and cattle aren't Australian icons. The game was in part a showcase of how awesome Australia is, and there isn't anything particularly interesting about sheep.