National Australia Bank has promised to “absolutely” compensate every customer who lost money as a result of a massive banking system outage on Saturday that lasted almost seven hours and affected up to nine million customers.

Shoppers and businesses across Australia and New Zealand went into meltdown on Saturday morning, with consequences varying from embarrassment at the checkout to, in some cases, thousands of dollars in lost revenue.

The outage disrupted internet, mobile banking, ATM and EFTPOS services between 7.50am (AEST) and 2.30pm on what is the busiest trading day of the week.

NAB confirmed the outage was caused by a series of failures from a power issue in mainframe infrastructure based in Melbourne, but the effects were felt nationwide and even across the Tasman Sea at its subsidiary bank, Bank of New Zealand.

NAB’s business executive general manager Cindy Batchelor, speaking at a press conference in the Melbourne suburb of Toorak on Saturday afternoon, said the full impact of the outage was still being tallied.

“We’ve had our branches on standby, we’ve also had our call centres taking calls from customers – but we don’t know the extent of the impact as yet,” Ms Batchelor said.

But a NAB spokesperson later confirmed to The New Daily that the bank had five million customers nationally and nine million customers internationally, including at BNZ.

Ms Batchelor said NAB would “absolutely” give 100 per cent of losses to customers and businesses affected.

“If the customer is impacted, out intention is to compensate them for their loss,” Ms Batchelor said.

“We would want our customers to come forward and provide us with the facts. If they don’t have that information, we will actually work with them to estimate what their losses might have been.”

Her reassurance followed a storm of enraged customers and small business owners sharing stories on social media on Saturday.

NAB apologised repeatedly through its social media channels during the almost seven-hour outage, but that did little to temper customer’s rage, with many threatening to take their business elsewhere.

Small business owner Lauren Knight, who runs a concreting company, told The New Daily she was unable to process payments for materials, but would still have to pay four sub-contractors a full-day rate despite the fact they had not been able to work.

“We are now going to have to pay for another day to complete what was supposed to be done today,” Ms Knight said.

“If I am not compensated I will switch [banks].”

The national bank’s business and private banking chief customer officer Anthony Healy apologised to customers in a Twitter video post just after 12.30pm.

Wearing a black T-shirt, Mr Healy apologised to clients “let down today” on behalf of the “executive leadership team and everyone at NAB” for the service outages.

“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.

“We’re sorry, and it’s not good enough, but rest assured we have our best and most experienced team on it and they’re working round the clock to get the services up and running as soon as possible.”

Our Chief Customer Officer Business and Private Banking Anthony Healy apologises to our customers who we let down today. pic.twitter.com/s8nqpmrj11

— NAB (@NAB) May 26, 2018

Many customers also expressed outrage at the disruption and NAB’s first apology on social media.

Beneath the apology, business owners told the bank they were losing thousands of dollars in business trade.

“We just saw $2500 walk out the door. But “Sorry this happened on a Saturday”, makes me feel all warm and fuzzy inside. So, ok…” one business owner responded.

https://twitter.com/MarkDCrompton/status/1000166762531323904

Customer Dave Younger said he was changing banks as a result of the banking meltdown.

“How many system-wide failures have you had now? Moving my accounts on Monday,” Mr Younger said in a Twitter post.

NAB wasn’t the only bank to experience service disruptions on Saturday, with Commonwealth Bank announcing merchant terminals were struggling to connect to the Optus Network.

A CommBank spokesperson told The New Daily the disruption was not related to the NAB outage.

We’re aware of an issue with some merchant terminals that connect to the Optus network. We apologise for this and we’re working hard to restore as soon as possible.

— CommBank (@CommBank) May 26, 2018