Hi towie, I think the difference is that Aboriginals have long been disadvantaged and have suffered in real terms because of that disadvantage. That doesn't strike me as the basis for a good joke.
I don't think there are hard and fast rules to this other than the test I mentioned in the previous post. The Irish and Jews for example have long been the butt of certain types of jokes that even though they play on racial stereotypes are acceptable to them in a kind of generic way.
