The deal with IBM follows a year-long review and forms part of a major programme of reforms to the bank's legacy core banking systems.

Under the agreement, IBM will take responsibility for NAB's mainframe computer, storage and hosting services. Big Blue will also maintain the bank's desktop fleet, printing, technology service desks and operating systems.

Speaking at the bank's annual shareholders' meeting, NAB's chief executive officer Cameron Clyne admitted that the bank is being constrained by the limitations of its legacy technology platforms. He says the recent afflictions validate a decision made two years ago to move to a new next-gen core banking platform.

NAB's processing problems first arose last month when an IT glitch delayed the processing of overnight payments and transactions and hit the bank's Eftpos, ATMs and other electronic banking systems.

In an update on its Web site, the bank says that is working to address inconsistencies in statements that continue to plague some customers.

However, the problems are likely to stretch into the new year, with the bank stating: "We aim to refund incorrect fees and interest charged by end of January 2011."