Am i still an Australian??

Was reading a thread started by Octalhex and decided to start one of my own instead of annexing a piece of his thread. I have a situation...perhaps someone could give me insight.
I too was born in Australia but i was there for only four years (the first four obviously) i then moved with my parents to the US for year and then to Belize. My mother is American but my father is Belizean. I have both American and Belizean citizenship, as far as i know, i am still Australian too...but i am not sure...my passport is expired (AUS) and i would just like to know what you think would happen if i applied for a renewal?

I am considering doing graduate study in Brisbane or in Auckland, and am guessing that it would be both easier and cheaper to study as an Australian than as an international. Anyone have any thoughts or experiences they would care to share?