Let me share an experience we had for the whole duration (4yrs) we stayed in Singapore...
We maintained a prepaid overseas sim the whole time and we use it to RECEIVE txt messages from our loved ones. It is very practical as local txt lang ang binabayaran nila when they send us an SMS. There is no roaming coverage charge for us on this as hindi naman namin ginagamit pwera sa pag-receive ng SMS.
However, if you use that to txt, call and even receive call - ayan super mahal ito...
Same concept applies here in Australia.