I would say that as they are not American, they don't know better. It's fair to say, I think, that attitudes about non-whites in Aussie culture is a bit less... advanced than attitudes about non-whites in US culture, and yes, I realize how low a bar that can be, but I think it's accurate. Most of the non-whites in Australia are actually natives (aborigines, or "abbos" as the slur goes).

Also, Australians are notorious for "taking the piss out of ya", what we might call "good natured ribbing" but they are less inhibited about it and seem to see it as a way to make friends rather than something you do with existing friends. They seem to respect if you can "give it back."

That's IMO and I'm probably stereotyping about Aussies here myself, but that does seem to be a trend. YMMV.