(heck, if it was on prepaid then ozzies could just take a telecom prepaid sim and use it in ozzie to get unlimited data for $180 per month. Pretty sure there are no unlimited mobile data plans in ozzie so it could be an option for heavy users IF it was allowed on prepaid.)
I use the Optus $2 a day prepay plan when I'm over there - unlimited calls to landline and mobiles and unlimited data. I haven't looked but I imagine there must be even better post paid plans available.
its only mobile internet on your handset. you can't use it for tethering or put the sim in a non-handset device (e.g. a dongle or mifi) which means if you want to use it for serious volumes of downloads it is still possible, but quite difficult..
"Sadly when you do read the Optus fine print in the Terms & Conditions it says the $2 charges exclude "mobile handset tethering and use of non mobile voice devices""
Lucky for me I've never read the T&C's :)
I tried a couple of years ago just to see what 'unlimited' meant and averaged close to 2GB a day torrenting over a week, with the handset tethered with no issues. I haven't tried it since but even now I get through anywhere between 100-500MB a day when I'm there (which on average is a week per month).