sbiddle:
It seems to depend on the card. Our office debit card (Westpac) needs the credit button.
If I understand correctly, it depends how the issuer has set up the card. Effectively it's two cards in one - using the EFTPOS network if you select CHQ or SAV, and using the V/MC networks if you press credit or use contactless.
Pretty much the same setup as you can have with many banks where you can have a current account linked to the CHQ and/or SAV buttons but CR(D) and contactless are linked to your credit account.
It may not work with contactless or credit if the merchant is EFTPOS only (does not accept credit cards)