Australian is an increasingly common euphemism for "cunt". To call somebody an Australian is to imply that they are the lowest kind of arsewipe, a thoroughly nasty piece of work, a sack of piss, a lowlife, an arsehole.
"Oh, that Tony Blair is a right fucking Australian, isn't he?"
Or, "Gordon Brown! He's such an Australian that in certain lights he looks like Shane Warne's arsehole twin!"