I received a Westpac Extras email with their latest offers. The first offer is "Get 10% cashback when you spend at Special Gift Cards." From what I can see, there is no list of approved cards, so you can buy any gift card on their site. Notable mentions are JB HI-FI, Woolworths, Amazon.
Offer Conditions as per email
Important offer conditions: Spend $1 or more in a single transaction on your Westpac Credit Card by either the primary or secondary cardholder. Limited to 1 cashback transaction(s). Cashback is limited to 1 card per customer, per retailer and will be paid into your account within approximately 7 days. Offer is valid for online purchases only. Offer limited to the value of $20 cashback. Offer eligible for online payments only. Offer cannot be used in conjunction with another offer.
TLDR: Must use a Westpac Credit Card in a single transaction. Limited to $20 cashback. Once time use.
Westpac credit card not debit MasterCard?