NAB says it will compensate customers "100 per cent" for any losses incurred during a nationwide outage that left people cashless and small businesses unable to process transactions on a crucial trading day.

Several of the bank's services were down for almost seven hours on Saturday, including internet and mobile banking, ATMs and eftpos.

The NAB disruption also affected customers across the Tasman, with NAB subsidiary Bank of New Zealand tweeting that its systems were down for part of Saturday morning.

NAB Business executive general manager Cindy Batchelor said the outage was caused by a power issue in the bank's mainframe in Melbourne.

"It's an incredibly rare event, but what it did is it took a number of hours for our technicians to be able to bring our systems back up," Ms Batchelor said.

"We still don't know the number of customers that are impacted.

"We've had an outage in terms of time — we know that that looks like. But we don't know the extent of the impact on individual customers at this time."

Customers slammed NAB on social media for the lack of notice about the outage.

Many people said they only found out about the problem when they were unable to pay for something, while others received an error message on their mobile banking app.

The error message that appeared to customers during the NAB outage on Saturday. ( Supplied )

Small business owners vented their frustration at not being able to process sales on their biggest day of the week.

The bank's tweet "sorry this happened on a Saturday" was cold comfort to those losing thousands of dollars in revenue.

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"We just saw $2,500 walk out the door. But 'sorry this happened on a Saturday', makes me feel all warm and fuzzy inside. So okay...," Twitter user @thecontactzero said.

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On Facebook Ehud Malka said: "As a cafe we lost about 25 customers today that went to other places that had working facilities on their premises. I am definitely moving away from NAB on Monday."

An Adelaide bakery directs customers to use cash due to a NAB outage. ( ABC News: Nick Harmsen )

NAB to assist calculation of losses

Ms Batchelor said NAB would fully compensate customers who could provide them with "the facts" about how much they lost.

She said those who did not have that information would be assisted by the bank to estimate an amount.

"If the customer is impacted, our intention is to compensate them for their loss," she said.

However for some customers the outage did not result in a loss — just inconvenience.

Beven Davenport told ABC News it was his last free weekend to purchase items for his wedding on Friday but the retailers he visited could not accept eftpos and he could not get cash out.

"It was unfair," Mr Davenport said.

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Earlier, NAB's chief customer officer Anthony Healy apologised for the outage in a video via Twitter — but that also did little to quell customers' chagrin.

"I want to apologise to those who are out trying to do their shopping on a Saturday morning, and particularly our merchants [who] are trying to do business and maintain banking services for their customers," Mr Healy said in the video.

"We're sorry, and it's not good enough."

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Twitter user @lachy_1234 responded: "Not good enough. Fix internet banking. You're a bank. You have one job."

Meanwhile, the Commonwealth Bank also suffered a brief disruption on Saturday to some merchant terminals that connect to Optus.

The outage lasted about two hours and came a month after the bank suffered a glitch that disrupted transactions for 24 hours.