I'm still not sure if it can be separately repaired (there are mixed messages about the iPhone 12 and 13 case glass being able to be serviced with a same-unit repair) but it would not be repaired under warranty if it's accidental damage. There are some rare cases where maybe the glass breaks spontaneously, but again that's very rare and would have to be limited to maybe a thin crack in the glass that shows no signs of an impact point.
But if if it requires a full replacement (with a replacement device that could be new or refurbished) then that's AUD $699 including GST. A replacement will require an exchange and they usually recommend that the customer back up the existing device before servicing. And if there's a replacement involved, they may allow the customer to use a Mac in the store to perform a backup and transfer to the replacement.
iPhone Repair – Official Apple Support (AU)
iPhone 12 repair costs in Australia
Out-of-warranty prices only apply to repairs made by Apple. Apple Authorised Service Providers may set their own prices.
iPhone 12 Other damage (out of warranty)
iPhone 12 Pro Max A$ 939
iPhone 12 Pro A$ 859
iPhone 12 A$ 699
iPhone 12 mini A$ 629