Here in the UK, networks have special iPad tariffs - for example, O2 charge £10/month for a 1 gigabyte allowance (that's not a fixed term contract, so you can cancel it with 30 days notice). O2 also include unlimited WiFi usage on any of their OpenZone networks, which are fairly extensive (I believe that many phoneboxes in large cities are now OpenZone WiFi points, and many Cafés etc. have a BT OpenZone network).
There are also Pay As You Go plans - for example, Orange charges 5p/MB which is better for light usage.
I assume that similar tariffs will be in place in Australia. It's worth noting you can't put a normal SIM card in the iPad - you need a special microsim, which means you can't just pick up a normal Pay As You Go SIM card.