In Australia we use the word hey in a few different ways but only very recently (Via USA tv shows like Friends and Buffy the vampire slayer) have we started using hey instead of hi as a greeting. It is extremely colloquial, used nearly exclusively by younger people. The word hey is used on its own to attract attention and I see it most commonly in a mixture of the two, a friendly attention seeking greeting, in texts (Hey, wt r u up 2?) and emails and informal chat between young people.

We also use hey as an exclamation of negative interjection (Hey! that's my apple!) interchangeably with Oi. There are a lot of expat Brits (pommies) in Aust.

Hey is also used as an identifier for a question where you are expecting an agreeable answer. (all done, hey?)

Or on it's own (What?) (Hey?) (Huh?)

Hey is a jack of all trades kind of word,