That is unfortunate. I applied online for a Low Rate Mastercard on the evening of the 20th, it was approved early afternoon on the 21st and I received the card on the 24th.
I am not an employee of the bank, but I will offer you some reasons why it may have been declined:
Do you have a clear credit check (ie: no adverse)? If not, do not pass go, do not collect $200.
Do you have existing credit card(s) with the bank? If so, do they show a high carried balance (ie: not paid by the due date and therefore incurring interest)?
What does the last 3-6 months of account history show? Are you always in the black or within approved credit limits?
If you are good for all of the above, I'd certainly suggest you follow this up with the bank.
Did your approval come through in writing? I received approvals for both my cards by email.
Was the decline within a few minutes of your online application? I have no first hand experience of this, but I think some customers may receive automatic approvals or declines.
My personal observation suggests the BNZ's approve/decline decision is heavily weighted on whether the bank believes the card holder can make the minimum monthly payment (capacity) and whether they are likely to (confidence).