Bank of New Zealand (BNZ) customers are waiting to find out what compensation they may receive after the bank's systems went down on Saturday leaving people stranded without access to their money for several hours.

The bank apologised to affected customers yesterday and said those who suffered financial losses would not be unfairly impacted.

The outage, which lasted several hours, was sourced back to BNZ's parent company National Australia Bank.

BNZ communications manager Cliff Joiner said it was caused by an outage at one of the Melbourne systems areas for the National Australia Bank.

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

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."

The bank was also working through how to best support impacted customers.

"This will include working with customers who experienced direct financial loss due to the outage to ensure they are not unfairly impacted.

"The outage was our mistake and we'll be working to make things right," Carter said.

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.