It's the wood. You can rank woods by the amount of strain energy per unit of mass that they can take before breaking. The higher this figure, the better the bow, because it'll store more energy for a lower mass in the limbs before it breaks, and the differences can be fairly significant.

The best bow woods are all at the top of this list, along with a few that should be good in theory but aren't used likely for other reasons (i.e. the plant just doesn't produce the kind of limbs you need to make a bow). The best Australian woods are fairly ordinary by comparison. On average they're hopeless. This isn't to say you can't do it (people do with spotted gum, which isn't a good bow wood), just that there are better ways of going about it if you want to hunt.

I'd be surprised if northern Australians at least didn't know about the bow, because there was frequent contact with Indonesia and people from PNG, all of whom practiced archery. There are oral traditions in places like Arnhem land testifying to this contact going back generations.