Most services have been restored after the major outage struck on Saturday morning, leaving NAB customers across Australia furious.
NAB says all customers are invited to speak with the bank about the losses they suffered in the "incredibly rare" outage and they will be compensated.
A screenshot of the NAB mobile app on Saturday morning. Source: SBS News
"We'll work with each and every one of them to understand exactly what happened to that particular customer," NAB Business executive general manager Cindy Batchelor said in Melbourne.
She said the bank was "sincerely sorry" to all customers affected.
"If there was a loss that was driven by the outage today then compensation will be provided."
The widespread failure struck about 7.50am on Saturday and lasted some five hours before most services were restored.
Stories rolled in from frustrated customers during the outage, including cashless shoppers stranded at supermarkets and businesses losing sales because they couldn't process EFTPOS payments.
Customers overseas complained of being unable to pay for things like taxi fares because of the outage.
Jaimee Lee said she only discovered the problem after filling her car with a full tank of petrol.
"I also have food shopping to do for my son's first birthday party so I'm panicking a bit," Ms Lee tweeted.
A power issue was behind a series of faults that hit the bank's mainframe in Melbourne, taking out the bank's major services nationwide, Ms Batchelor said.
"It's an incredibly rare event and it took a number of hours for our technicians to be able to bring the systems back up," she told reporters at NAB's Toorak branch on Saturday afternoon.
"It has had an incredible impact on our customers and for that we're incredibly sorry," Ms Batchelor said.
"It's not something we're proud of because today we failed to deliver the service we aspire to deliver."
No personal data was compromised during the system failure, Ms Batchelor said.
Customers of the NAB-owned Bank of New Zealand also faced similar problems on the other side of the Tasman Sea.
NAB executive Anthony Healy took to social media to issue a video apology for the outage, saying it wasn't good enough.
"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 around the clock to get the services up and running as soon as possible," Mr Healy said.
