National Australia Bank (ASX: NAB) has confirmed it experienced a major IT outage resulting in payments made overnight not appearing in potentially "millions" of customers' accounts.

"NAB wishes to advise that we have experienced some issues overnight which have delayed the processing of some customer payments and transactions," the major lender said in a statement on Thursday.

All types of payments had been affected by the four-hour systems outage, according to a bank spokesman.

NAB said it has been working as quickly as possible to rectify these issues and delayed payments and transactions have started to be progressively processed.

"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."

"Customers should also be advised that there may also be some delays for NAB EFTPOS, ATM and other electronic banking facilities."

The bank said these issues have resulted in a significant increase in traffic to NAB customer call centres, which has also caused some delays.

At 11:20 AEST, NAB shares were down 2 cents or .09 per cent at 23.43.