The newly introduced TPG's "Pay As You Go" mobile phone plan has got to be the cheapest one around. For just $1/mth, you are entirely on a pay-as-you-go plan and the rate is far cheaper that any prepaid plan you can find. The cost to make a call to a landline or a mobile is 10c per min, plus 10c flagfall (computed in seconds).
In comparison with the next cheapest rate, VOIP service provider MyNetFone charges 20c per min, no flagfall (computed in increments of 1 minute).
Next up, I think reseller Lebara gives a pretty good rate too: 15c per min, plus 25c flagfall (in increments of 1 minute). Although significantly higher than TPG's, it is a lot cheaper than other prepaid mobiles (e.g. Optus).
If you think flexicaps are cheap, think again. Vodafone's $29 cap gives you $150 call credit, plus $150 vodafone-to-vodafone talk time. $150 sounds like a lot, but you have to pay 80c per min, plus 39c flagflall. If you make 2-min calls each time, $150 will last you 75 min. In comparison, with TPG's plan, $29 will give you 96 min (2-min calls, at 30c per call).
TPG's plan does not require you to sign a contract, or commit yourself to a $29/month usage. It is truly the cheapest mobile rate in Australia. In Australia we are all paying far too much for our mobile phone usage, so spread the word around! I have already swtiched. Just tel. TPG at 13 14 23.