At checkout on desktop version, your cart should read:
Qualifying offers:
GC promotion
This means that you have successfully applied the promo code for the first time. If it is giving you an error, it means that you have entered the code already
The first 10,000 customers of Amazon.com.au who enter the promotional code “AMAZONGC” in the “Gift Cards & Promotional Codes” box at checkout after purchasing $75 or more of Amazon.com.au Gift Cards in a single order will receive a $5 promotional credit that can be used on a future eligible order (“Offer”). You must use the $5 promotional credit by 11:59PM (AEST) on 31 May 2023. This Offer is available between 12:01AM (AEDT) on 21 November 2022 and 11:59PM (AEDT) on 30 November 2022. Your promotional credit will be automatically applied to your account used to make the purchase within two (2) business days after your order is shipped (for physical gift cards) or within two (2) business days after the order is completed (for electronic gift cards).
Credit: bargainsgrabber
Unfortunately, as a NZ customer, you'll probably have to talk to live chat to get the credit added to your account. I've had no issues with that part, but it is a pain.
https://www.amazon.com.au/blackfriday