Wicked - questions regarding the show and tickets

Hi! I'm currently trying to convince my boyfriend we need a holiday in Melbourne, and while there I'd LOVE to go and see Wicked. I've never seen it, or even heard a recording, but from what I've read it sounds like a great show. Part of convincing my boyfriend to go is keeping the costs as low as possible. I've heard there's a 'lottery' for tickets at Wicked, but I just wondered how that works? I'm guessing it's a day-of-the-show thing, and a fairly big risk? I'm also wondering if anyone can tell me whether Half-Tix in Melbourne are selling Wicked tickets yet? Thanks! Margie