What is the response of at!gstatus?
Have you entered correct APN in at+cgdcont?
Other user can make it work under australian network:
BTW, have you tried the latest FW?
https://source.sierrawireless.com/resources/airprime/software/rc76xx/rc76xx-firmware-release-9,-d-,6,-d-,1/#sthash.TSMrq0GU.dpbs