Do Debit mastercard numbers change when replaced by a new one? Specifically in Australia.?

No, the numbers don't change, and you shouldn't be charged a renewal fee either, although I'm not sure about Commonwealth Bank of Australia specifically. Come people have standing instructions regarding payments with their banks and other service providers, and the card numbers cannot change. The only thing that's changed in your new card is your expiry date.

This is a gift card that works similarly to a debit card which is connected to your checking account and deducts the funds from it. I believe in this case, it is for the cashier so that they can code the transaction as a debit card. Gift cards are not reloadable. Once the balance is used, it can be discarded. It also has limitations such as you can't use it like an ATM to withdraw cash; if you purchase an item and return it, you cannot get cash in return (they will refund the amount back to your card); you cannot pay bills with it, and I know with the American Express gift card, if you do not use it after a year, the balance is subject to a $2.00/mo. charge thereafter until it is used. You really need to read the "fine print" to know all the rules it is subject to. Other than that, you can have fun and spend your Christmas present on what will make you the happiest!