Most (all) Australian carriers either don't lock or have unlocking available for a nominal fee (sometimes free) for the phones that they sell on a contract. If you think about it this makes sense - they already have your money, if you don't want to use their network, hey costless revenue. Of course in most cases the handset is not "subsidised" per se, just offered on credit. That's not to say that the every sans handset plans are actually good value.
Prepaid is another matter as the handsets are actually sold below cost - though even there all the carriers offer unlocking for semi-reasonable fees that often decrease over time.
Mandatory number portability probably also helps consumer (carrier) mobility. The advent of pentaband devices removes the last technical barrier.