Hey! Fellow Aussie here.
There's quite a big following of cosplay and conventions in Melbourne.
Just this weekend gone you've missed out on the Armageddon pop-culture convention, which of a normal year my partner and I head down from Sydney to go to.
If you've got Facebook, there are quite a few cosplay groups out there, but as a starting point, head here and join the group: https://www.facebook.com/groups/australiancosplay/
And then chuck up a post asking for links to any specifically Melbourne-based groups.