I got to know that the Allphone shop in Chinatown, Adelaide is selling Optus $2 prepaid offer as unlimited mobile broadband. They bundle sale with all kinds of Optus labeled USB modem and Wifi Modem. Since a lot of students are using it already without problem, I reckon it's feasible doing so to save money.
Just buy an Optus prepaid SIM anywhere. Activate with the $2 day offer. Put it in 3G modem or mobile phone. Possible wifi tethering.
The APN is 'yesinternet'
The $2 is only charged when first used on that day.
The other benefits are unlimited calls and sms.
Please note no P2P is allowed. However if you get banned, just swap to another prepaid SIM.
500mb is the same as unlimited on the Optus network…