Well im from the U.S, but i feel like those dresses are a little extream for a sschool event. Any dress that looks like that is probably going to be very expensive. If i were you I'd just go with a short dress or a semi longer dress (not floor length) & just spice it up with accessories and a cute hairstyle and make up. I wish i could find some stores for you but...i have no clue whats in Tasmania hahaa :)

Ha ha, we have similar taste in clothes. I love old vintage stuff too. It's hard for us people in the U.S. you point you to the right sites and stores, so there's no point in me showing you a fabulous dress that would cost more to ship than it would to buy. But check with my friend Babiedoll. She's Australian but not from Tasmania and she has fabulous taste. She can probably point you to some great Aussie sites. Here's a couple I saw that have really cute retro clothes.

http://www.prettydress.com.au/

http://www.beautvintage.com/evening-party-dresses-c-42.html?osCsid=cf7a2316d25bf08db7b601f4420bb898

http://www.vintageclothing.com.au/
MusicQS has some great ideas and I understand her point about a short dress, but if your heart is set on a longer, retro dress (like Marilyn Monroe or Audrey Hepburn, what I would do is go on the Internet and look up vintage clothing stores where you live and go check them out, maybe with a friend. I bet you'll find something exquisite! Looking at the stuff on some of the sites, I came across this dress that I think is darling but it's not cheap. Good luck

Thanks for your answer, i guess you could say it is a bit "over the top" but here in Tasmania it's the only time to really shine! :)

Thank you for your reply, sorry to say that it wasn't really helpful, but the effort you put in was great. :)