Westpac says it has fixed a problem that blocked some credit card payments on Sunday.
The bank says the hold up lasted for a couple of hours.
Westpac's problem came to light when a customer found payments on his credit card were unexpectedly declined.
He then attempted to use his card at an ATM but the machine swallowed it.
The customer says he was told later by a Westpac call centre there had been many cases like his, stemming from a computer upgrade that produced some unexpected glitches.
Westpac management are not confirming that a difficult software upgrade was the reason for the problem.
But a spokesperson says there was a definite increase in complaints to call centres about bank cards, and the problem took about two hours to fix.
She says the problem was confined to a few hundred customers.