The big four bank was besieged by multiple failures to technology systems across its network on Monday.

NAB's mobile banking app, desktop internet banking, merchant terminal and trade services were all impacted by the outage.

Group executive of business and private banking Andrew Irvine described it as a "tough day" for customers.

"Days like these are exceptionally hard; none of our colleagues ever come to work wanting to let their customers down," Mr Irvine posted on Linkedin.

"For this, we are incredibly sorry. We know, we understand, we feel the impact it has on our customers and their businesses.

"Our colleagues are doing everything they possibly can to minimise the disruption and get our services back up and running."

NAB Trade, NAB Connect and NAB Merchant terminals were the first to come back online on Monday afternoon.

The bank later said customers could once again login to desktop internet banking and its mobile banking app.

"We continue to work through some functionality issues," it tweeted.

"While fast payments are unavailable, you can make traditional payment transfers using BSB details."

Australian Associated Press