Amaysim's $39.90 Unlimited SIM deal is a popular choice amongst Lifehacker readers, offering unlimited Australian calls and texts plus 4GB of data. It's already one of the cheapest mobile plans on the market, but right now you can sign up for the service and pay just $10 for the first month.
Just use the promo code LIFEHACKER212 when signing up, and you'll pay $10 for the first 30 days instead of $39.90. This only applies to new SIM activiations, and you have to activate online. Full terms and conditions are here. The deal only runs until the end of February 22, so get in quick if you're keen.