NAB says it will compensate customers for a nationwide network breakdown.

The fault knocked out internet and mobile banking, ATMs and EFTPOS, leaving people stranded and cashless, while some small businesses were unable to process transactions for several trading hours.

The outage was traced back to a power issue in the bank's mainframe in Melbourne, according to NAB Business executive general manager Cindy Batchelor.

“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.”

NAB was taken to task on social media, with customers and small business owners feeling less than placated by the bank's tweet; “Sorry this has happened on a Saturday”.

NAB says it will fully compensate customers if they can provide them “the facts” about how much they lost.

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

For most customers, the outage resulted in inconvenience, not financial loss.

Additionally, some Commonwealth Bank EFTPOS terminals that connect to Optus were also knocked out on Saturday.