Worked for me at checkout. Hopefully see the credit soon (within two business days).
To qualify, you must: (1) be one of the first 5,000 new eligible customers to purchase $50 or more in Amazon gift cards in a single order at Amazon.com.au until 11:59pm AEDT on 23 October 2022, and (2) enter promotional code "NEWGIFT" in the "redeem a Gift Card or Promotional Code" box at checkout. Promotional credit expires 11:59pm AEDT on 31 March 2023.
New Eligible Customers means customers who have not previously purchased Amazon Gift Cards on Amazon.com.au or the Amazon App. If a new Amazon.com.au account is created, it may take up to 48 hours for the promotional code to be successfully applied at checkout.
Promotional credit will be automatically applied to your account within two (2) business days. A confirmation email will be sent once the promotional code has been applied to your account. Promotional credit can only be used for purchases of eligible items shipped and sold by Amazon AU and will be automatically applied during checkout. One promotional credit per customer.