This was old issue in AU bank that existing credit card holder can't open Zero balance online account but that was fixed in Jan 2024.
I remember I had AU credit card from Nov-2023 and I applied for Zero balance account in Dec-23 and got error. Customer care and tweeter support team said that since I already have existing credit card from AU, I can't open online zero mab account.
In January 24, I just tried again and surprise surprise 🫢 account got opened in 5 minutes. Even no initial funding was asked
After 48 hours I added fund in account, 150+GST deduct and Debit card issued.