Thanks for the responses so far guys!! 
I'm not so much concerned about having the cops rock up due to a rowdy crowd. Just more concerned if people drink irresponsibly, and well munt and become very drunk. Because that's when you'd need to call their parents, the whole situation becomes "real", and depending on their parents reaction could cause issues.....
So, if I get them to bring their own alcohol, I needn't require notes from their parents or anything?
You need
1)Notes from parents (email which clearly states the kids name, the adult name, the permission given and the date and legally valid and normally the best option-seeing it's harder to get into an email account than it is to write a forged note).
2) Supervision by a parent of suitable guardian (I don't think the guardian is allowed to be drinking with the kids, cannot confirm though).
3)It's illegal for kids to bring alcohol with them in a public place, so to comply with the law, adults would have to buy a bring the alcohol.
Also just because you comply with these doesn't mean you can't get done for other related things. e.g. if people are rowdy and the guardian isn't exercising proper supervision, the guardian could probably get done for any damage etc.
[Oh and also the law is perfect in theory, it's only when kids fuck it up by committing forgery it goes into disarray, the vic gov have already changed the law to deal with stupid kids (i.e. kids who consistently and willing skirt the laws) so don't be surprised if it's changed in the future and the "fun" is taken away from everyone]