Usually contract phones in Malaysia are not locked if I'm not mistaken, contract phones in Aussie are mostly not as well, voda locks them but you can unlock them easily from their website for free for most phones. Since the S3 is quad band 3G, there's also no problem using them with voda or Telstra's 3G networks that uses the 850mhz spectrum.

I'd just go and buy it outright in aussie though, grey importers like kogan,mobicity etc. are having some cheap price wars, might be cheaper than getting on a Malaysian plan. Snoop around ozbargain with the occasional sale from some random grey importers beating everybody else's price lol.