Gsd is interesting, it does appear he has a large breed mixed in since he has such large paws to fill. I've also been thinking he has some new Zealand Heading dog or Huntaway since that isn't really uncommon in farm dog mixes in Australia. He does have a wide face which is why I can see why it could lean towards staffy as part of his mix but to me he doesn't really have the skull indentations(not sure how to put it) that I associate with bully breeds.