It appears that it also depends on what are of Australia you reside in as to when it will be celebrated:

http://www.statusquo.org/birthday.htm
and
http://goaustralia.about.com/cs/eventsfestivals/a/queensbirthday.htm



>I'm in the process of updating our application's holiday table for Australia and there's one holiday that is giving me trouble in terms of coming up with some kind of calculation methodology: Queen's Birthday.
>
>On first glance, it looks like the holiday falls on the second Monday in June, but 2005 blows that theory. Is there anyone who could please inform me as to the calculation rule to determine when this holiday is observed from year to year?
>
>Laterness,
>Jon