Just remember that 3 is only in the metro areas, not in rural areas (outside of melbourne / sydney I believe).
Another course of action would be to go with a $0 contract with somewhere like exetel mobilewhich means you only pay for what you use. They use the vidafone / optus network so their coverage is quite good (if that is an option for you). This is my ISP, I do recommend them.
There is also Optus (see the optus website here ) and their prices are acceptable (maybe patchy, but I have never seen this first hand), and the speed is good according to reviewers, here is their basic price structure:
Otherwise places like dodo have prepaid wireless, but I'm not sure about their pricing.