It sounds like the whole of Australia has no 16GB prepaid iPhones. At least not in Melbourne!
I waited in line at 6 and as soon as I got in, I was told that of the 5 prepaids they had in the store, all were 8GB. I went to almost every Optus dealer in the CBD and they either had NO prepaids to begin with, or didn't have 16GB delivered. There was also a Telechoice on Swanston St that didn't even have ANY stock delivered yet (not even contract iPhones) - they were expecting it to arrive at lunchtime. On the way home I dropped by the Camberwell Optus shop, same deal - no 16GB.
At least when I visited all of those stores, I put my name down for pre-order/raincheck, and I was either the first or one of the first on the list, so hopefully I won't be waiting too long (at least next week to 3-4 weeks). I can handle the wait so that's no problem.
But it was just amazing discovering how ill informed most of the staff were. And also that most stores had about 100 in stock, but almost all were for contracts. Understandably, Optus wants to take advantage of their superior plans, who would blame them? Still, it's odd that there were absolutely no 16GB iPhones on prepaid.
For those who got one today, enjoy!