The IT issues resulted in a spike in telephone banking, which added to the problems, resulting in call centre delays for frustrated customers.

The bank says it is working to rectify the problems and delayed payments and transactions have started to be processed.

Says the bank: "This will take some time and while we are hopeful all delayed payments will be able to be processed by later today we will provide further updates as things progress. NAB apologises for any inconvenience or confusion these issues may have caused our customers."

The giant Australian bank experienced a similar outage in 2008, that left thousands of customers unable to access their salary payments.

Update: At 6pm Australian time on Friday, NAB posted the following update: "Payments from 24 November have been processed. In addition, due to remediation work on completing this processing overnight, some payment processing from 25 November has also been delayed and is currently being processed today and during Saturday 27 November."