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.
NAB executive general manager Cindy Batchelor said their company would look to refund customers affected by the outage.
"If there was a loss that was driven by the outage today, then compensation will be provided to customers," she said.
"We'll work with each and every one of them to understand exactly what happened to that particular customer."
NAB customers and businesses were left unable to access bank accounts and use Eftpos machines after the services went down.
- Additional reporting news.com.au
