In Singapore, it costs only US$0.08/min (8c) to receive calls, or US$0.41/min (41c) to make them within Singapore or back to Australia. This was the cheapest of all the global SIM cards we looked at for Singapore.
Buzzroam is a Hong Kong-based company providing SIM cards issued by a Belgian mobile network. However, it provides an Australian phone number free of charge that your contacts can call, which redirects to your mobile wherever it is in the world, so the calling party doesn't have to pay international call rates.
You can also divert your regular mobile number to this number (at standard call rates, though for many people this will be included in their calling cap allowance at no additional cost).
The upfront cost for a Buzzroam SIM card starts at US$35 ($35.55) which includes US$10 ($10.16) calling credit. Postage is only US$4 ($4.06) to anywhere in the world, including Australia.