Phone locking used to be a standard industry practice the telcos used to stop customers moving to rival networks, but this is no longer the case. The vast majority of phones sold in Australia now are sold unlocked.
The exception is prepaid phone handsets. These phones are heavily discounted by the telcos and phone locking makes sure you stay long enough to make up for the discounted price.
Typically you will be required to pay an unlocking fee within the first 6 months of owning your phone. After this unlocking is free, or much cheaper, depending on which telco you are a customer of.
If you want to unlock your prepaid phone, contact your provider.