She first noticed a problem with her card when she tried to buy a coffee this afternoon.
"I inserted the card and it said there was a problem with the chip. I tried about four times before giving up and swiping it.''
About 10 minutes later, at a supermarket checkout, the woman's card wouldn't work.
"It kept telling me there was a problem with the card. It was getting embarrassing and I thought I was going to have to dump my groceries at the checkout and leave,'' she said.
After about five attempts, the card was accepted.
It was only later when she checked her account online that she realised she had the same amount taken out of her account twice.
ANZ spokesman Stefan Herrick acknowledged this would not have been an isolated incident.
"This happens under some circumstances. We expect it to be a small number of customers but the accounts are all going to be looked at overnight,'' he said.
"When anything like this happens the funds are replaced and no customers will be out of pocket. It will be fixed within the next 24 hours, maybe the next 12 hours.''
Mr Herrick would not say how many eftpos terminals or customers were affected, citing commercial sensitivity.
However, he said it was a small proportion of the bank's customers.
He said ANZ's call centre had been busier than usual today, with merchants trying to find out why they could not process sales.
Some ANZ customers took to Twitter to vent their frustration.
"Finally some exposition for this morning's coffee-acquisition mission: ANZ eftpos cards malfunctioning,'' said one user.
"Luckily I don't shop on a Sunday,'' said another.
ANZ said ATMs were not affected and apologised for any inconvenience.
