A word used by arrogant australian bigots. It essentially means unpatriotic. It has the same basic meaning as 'un-american' except that american and australian bigots have differing opinions on what makes a patriot. Calling a woman un-Australian is normally considered to be a light-hearted joke. The word is normally used by men, against other men. Calling an australian man 'un-australian' is questioning his man-hood.
Some things considered un-Australian:
Failing to drink beer socially.
Failing to know or care about sport, specifically football.(as opposed to soccor)
Knowing the second verse of the Australian national anthem.
There are three men standing around a BBQ, one offers to go inside and grab some beer. Another man states that he'd rather not have a drink. The third says "Jesus mate, that's downright un-Australian, what are you? A poof or something?"