I joined a few clubs/societies last year, some of which were good but others not so. It depends on what you're into.
I highly recommend joining MASS (Melbourne Arts Students Society), SSS (Science Students Society) and CSS (Commerce Students Society), which hold regular BBQ's with free beer and sausages. These BBQ's are definately good places to chill with friends in your spare time. By memory, I think they cost like $2 per society (I think the CSS is abit more expensive) or something so it's definately worth it and you usually get other discounts for pubs around uni too. Also they e-mail/txt you upcomming parties that are being organised which is handy if you want to go to them, because they generally sell out very quickly and knowing when/where tickets are being sold is good.