"G'day mate" is probably the most important phrase to use, and I'll put my hand up now and say that vegemite is like axel grease.
Don't ever ask for a Fosters. It's not a good beer, and you will be picked for a newbie immeadiately. (why do you think we export it). When in Sydney ask for a schooner of VB, or a schooner of NEW. In Melbourne ask for a pot of Draught, or VB. In a good bar however, you should always ask for a "Crownie"
A "Dunny" is a toilet. (Barfly's will tell you there's no toilet, only dunnies)
"Footy" Everyone will talk about it. In Sydney it's Rugby League. In Melbourne it's AFL.

Where abouts are you going? Between us we should be able to point you in the right direction.