I agree with Cassie. I can see where you'd think she's an AmBull, but if that breed is so rare is Australia, I think it's extremely unlikely. I honestly think she looks like a boxer/bullmastiff.
It's like those shelter ads I've seen on petfinder that label a dog as an "Otterhound Mix." The dog may look a bit like one, but given the extreme rarity of the breed, I doubt that's possible.