The Bank of New Zealand (BNZ) says it has received 35 claims for financial loss linked to its major outage last weekend.

The bank's online banking, eftpos, credit card, ATM and branch services went down last Saturday for several hours cutting people off from accessing their accounts and prompting hundreds of customers to post complaints on the BNZ Facebook page.

BNZ promised to ensure customers were not out of pocket for the outage which has been blamed on a power failure in Australia affecting the data centre run by its parent company National Australia Bank.

A spokesman for BNZ said so far 35 customers had been in touch and 31 customers had had their complaints resolved.

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"We are working with the others to put things right for them."

Customers have questioned how a power cut in Australia could affect the New Zealand business so badly.

The BNZ spokesman said initial investigations suggested the problems were related to a power issue at its Melbourne Data Centre where the "backup power options did not operate as expected."

"Backup power systems are in place and investigations are continuing into how the power failure occurred and why backup power systems didn't kick in."

However, he said the impact of the outage was limited due to multiple back up processes the BNZ had with the payments industry.

"These systems ensure we provide the ability for the majority of card transactions to be accepted during events like the one experienced on Saturday.

"Those backup processes ensured that the majority of BNZ cardholders could use their cards through retailers' point of sale terminals."

Some customers complained of being left with a trolley load of groceries unable to pay while others could not pay for petrol leaving them stranded.

The bank has apologised for the inconvenience caused to customers.

It has not offered to compensate customers who were put out by the experience but has said it will treat each customer complaint on its merits.

"We apologise to any customer who feels their experience with us was not acceptable.

"We encourage them to contact us and we'll treat each case on its merit."

The bank said it would be reviewing the situation to ensure it didn't happen again.

"This was an incredibly rare event – and we are constantly working with our technology to ensure we can provide robust systems and processes for our customers.

"In the rare occasions when we do have an event like this there is a review and we apply what we learn to make sure our systems are even more robust."

The bank also came under fire for the way it communicated the outage with customers with some questioning why the bank didn't text message them directly or give more details about what services were affected and when they might be fixed.

The BNZ spokesman said it had kept its customers informed by using social and digital channels and through the media.