KevinL: For what it's worth, I just got back from Melbourne - the Optus at the airport would only sell the $2 days SIM card with a minimum 30 day topup (so $60), or Prepaid Social ($50). Went for the Prepaid Social option - the rep swapped out bundled international calling/texting for an extra GB of data, which gave me a bit over 4GB in total.
Would have preferred to just pay for 6 days worth of $2 days, but the rep was convinced that wasn't an option (and I couldn't be bothered investigating further).
That also happened to me in the same store. I let it slide as I just got off my flight and had other things to get organised but contacted Optus later on and confirmed my suspicions were correct. However, I would have to go back to the store I purchased it from to get a refund. I wasn't left very impressed by the whole thing. As for coverage, I had no issues at all whilst in Melbourne and travelling the Great Ocean Road.