Some "The Good Guys" stores have $5 Virgin Prepaid SIM cards with $20 of credit included in their bargain bins, I know it is available at their Port Macquarie and Coffs Harbour stores. The packaging doesn't say there is $20 included credit, probably why we can get such a good bargain!
There are also some Telstra Prepaid SIMs there.
If you don't manage to find one, don't worry. Virgin also sell $5 Prepaid SIMs with $10 of Motor Mouth credit included.