Redbull mobile have just started operating in Australia (late 2011) and I found they offer very competitive plans. I purchased a years coverage (they use Vodafones network which I have not had a problem with) for a dollar a day with a phone for an extra dollar outright. I haven't been able to fault the service after 4 months use with helpful support staff and an easy to use website for managing your account and recharging (if you choose the prepaid version like me). They also offer some fantastic competitions such as a trip to the Formula one in Melbourne looked after by Red Bull Australia in their corporate hospitality suite which I was lucky enough to win this year. For the money I paid they provide unlimited calls and sms's within Australia to pretty much any phone and a genourous pack of monthly data (5gb a month) that has kept me extremely happy

Approximate cost: $365