If you're looking to have your phone completely off-contract and pay as you go, Optus Australia are promoting their business to all prepaid customers, throwing in some bonus' if you walk down that path - for a limited time only.
As of today Optus will be offering an upgraded $30 AU prepaid service, which gives you 30 days of access, 1.5GB of data, 350 call minutes and $5 of extra credit for premium services (like MMS). In total this is a 500MB improvement on their previous $30 AU offering.
If you're looking to spend a little more, $45 AU will get you unlimited call minutes, 3GB of data and $10 of bonus credit. This is surpassed yet again by a $60 AU offering which will gather you unlimited minutes yet again and a beefier, 6GB of data.
This option is only available until the 19th of April, seeing customers who sign up before this date able to keep it for life - otherwise the old plan will be back on offer for all newcomers.
Also running on the Optus network, Amaysim offers a similar plan - seeing $44.90 AU a month grant you 5GB of data, unlimited calls and text messages over 30 days of access.