I have just signed up to iSIM, a Optus GSM network reseller that's owned by SIMplus (which is part of Optus, which is of SingTel). Who can pass on their special deal? $1 for a prepaid mobile phone start up kit that includes $20 of call credit delivered to your home. 39 cents per minute at per-second billing. No stupid flag falls that everyone else seems to be charging. 15 cents for each SMS message. Moreover, it provides call history on line at their website, 1 minute after you press the 'End' button on your phone. Prepaids usually don't come with any kind of call statements, so with on-line statement you know where the call credit has gone.
The down side is that it does not have any discount for intra-network calls, nor special rate period like Optus' Yes Time. 39 cents per minute flat rate 24/7 - a bit like the original Vodafone's No Plan but cheaper.
So my number will be ported from Vodafone once I received my kit. And my Vodafone friends - don't call me excessively as it would not longer be discounted.