Six days after a system glitch that delayed millions of transactions, National Australia Bank (ASX: NAB) said on Tuesday it has corrected all affected accounts.

Balances of the remaining 19,155 accounts are now up-to-date, according to the major Australian lender.

"This means that all customers are now able to transact as normal," NAB said in a statement.

NAB's system is believed to have broken down last Wednesday at 4am, when many overnight payments are automatically processed. The outage, caused by a corrupted file is said to have affected both retail and wholesale transactions.

The bank advised yesterday that the vast majority of affected accounts were resolved over the weekend and operating as normal as of Monday.

"NAB again wants to extend our sincerest apologies and thanks to our customers for their ongoing patience and understanding as we have worked to resolve these issues."

The bank reiterated that customers are not left out of pocket as a direct result of the delays which have occurred in recent days and said that any fees charged as a result of the computing debacle will be refunded.

"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."