Bank of New Zealand could be doing more to compensate customers who lost access to their money on Saturday for several hours, says Consumer New Zealand.

The BNZ has said it will work with customers who experienced a "direct financial loss" outage to ensure they are not out of pocket.

But those who couldn't pay for groceries or petrol because their cards wouldn't work may get nothing.

A BNZ spokesman said it would work with any customer who experienced direct financial loss.

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"We will work with them to ensure they are not financially impacted. Customers should get in touch with us and we will look at each case on its merits."

But Jessica Wilson, Consumer New Zealand's head of research said the bank could be doing more to compensate its customers.

"There will be a lot of ordinary customers who have been affected."

An offer like a small amount of credit as compensation for the inconvenience of the service being out for so long would be appropriate, she said.

"Most of us rely on using cards."

Wilson said people who only had accounts with the BNZ would have had no other options for paying by card.

Wilson said she wasn't aware of any other major outages by the banks recently.

Other industries like the power and telecommunications industries have compensation requirements if they don't fulfill their obligations for a certain amount of time.

Wilson said any changes around that for the banks would have to be introduced by the Reserve Bank of New Zealand or the Financial Markets Authority.

But she did not believe there were enough problems to justify an intervention at this stage.

The BNZ spokesman said the problems were caused by an outage at one of the Melbourne systems areas for the National Australia Bank - the bank's parent company.

He said the outage caused a series of failures on both sides of the Tasman.

Yesterday chief customer officer consumer and wealth Paul Carter said a power failure was the cause of the problem.

"Our immediate priority has been to get the system back up and running and I'm pleased to say it was back to normal Saturday afternoon."

Hundreds of BNZ's customers turned to social media to vent their frustrations on Saturday.

Some customers spoke of the embarrassment of getting to the supermarket check-out having to leave a trolley full of groceries behind as their cards were declined.

Others were stuck unable to travel because they couldn't pay for petrol.

While one mother said she was unable to purchase the food and drink for her six-year-old's birthday party.

Customers questioned why the bank had not informed them of the outage by text message.

BNZ first reported problems around 10am on Saturday.

An incident report on the BNZ website at 10.04am said it was investigating why customers were unable to login and access BNZ internet banking services which include mobile banking and client funds services.

By 1.19pm it reported that the problems had been resolved.

But some customers were still reporting problems with access to internet banking yesterday.