National Australia Bank (ASX: NAB) says it has identified and fixed the problem which caused the payment and transaction processing delays it has experienced in recent days.

According to the major lender, issues have been resolved for the vast majority of its customers.

"All instances of duplicate and multiple transaction entries which may have appeared in some NAB customer accounts in recent days have also been identified and the majority of these have now been corrected so that transactions and balances for these accounts are now accurate and up to date," NAB said in a statement on Monday.

Almost 20,000 customers, however, are to be kept waiting.

"Some additional work still needs to be done on around 19,155 remaining accounts where these instances of duplicate or multiple transactions have occurred. We have a process in place to do this and will continue to work to bring these accounts up to date throughout the course of today and overnight tonight," NAB said.

The bank stressed that customers are not left out of pocket as a direct result of the delays which have occurred in recent days.

"A process will be put in place to identify and put right instances where NAB customers may have inadvertently incurred fees, interest or other charges from the bank as a direct result of these delays. We will proactively undertake this work over the coming weeks without the need for customers to contact or notify us."

"Where customers have been charged a fee or have incurred a cost from another institution as a direct result of these delays, we encourage them to contact us so that we can work with them to also put things right."

Delayed value exchanges from NAB to other banks have also been processed over the weekend. This means that other banks can now also begin to process any salary or other payments to their customers which may have been impacted by the processing delay.

"We can assure every NAB customer that their accounts and other banking products are completely secure and that all account balances will be brought fully up to date."

NAB said it will continue to provide updates as more information becomes available.