Go back two years and the safe presumption was that a prepaid mobile plan would have a lousy data allowance, but things are changing. Savvytel's Savvy Unlimited plan includes 4GB of data as well as unlimited calls and texts to Australian numbers, and costs $38 a month.
That $38 price point makes it marginally cheaper than Amaysim's similar Unlimited deal, and just marginally cheaper (Amaysim is $39.90). Both use the Optus network, charge 5 cents per megabyte for excess data, and don't require any long term commitment. (Boost Mobile also has a fairly similar $40 deal, but only includes 3GB of data.)
Right now, Amaysim is offering the third month free for new customers who sign up for the Unlimited plan, which would ultimately make it considerably cheaper than Savvytel for any period longer than two months. That deal expires on January 5, however.