Combine the 15% bonus cashback with 6% standard cashback, brings up to 21% cashback in total
According to ShopBack:
The Cheesecake Shop gift cards can be used both online and in store.
Currently for online orders the order needs to be equal or less than the gift card amount. Split payment is not available for online payment. For orders more than the gift card value can be redeemed in-store.
Note that Shopback doesn't have any customer service these days, so if your Westpac cashback doesn't track (which has happened to a lot of people recently), you won't be getting your money.