The problems first arose on Tuesday when an unspecified IT glitch delayed the processing of overnight payments and transactions and hit the bank's Eftpos, ATMs and other electronic banking systems.

The bank has denied claims that the crash was caused by a routine IBM mainframe upgrade that failed to complete. Instead, it has blamed a "corrupted file in the processing batch" as the source of its woes. How the file was uploaded to the mainframe - and by whom - has not been revealed, although the bank asserts that it was not the victim of sabotage or an outside attack.

As the IT issues escalated, the bank experienced a spike in telephone banking, which added to the problems, resulting in call centre delays for frustrated customers.

Yet despite the bank's repeated assurances that it was on top of the snag, the problems multiplied for customers going into the weekend with payments into and out of their accounts appearing and disappearing at random.

NAB was forced to open branches throughout Sunday in order to reassure customers as cash machines and call centres buckled under the strain.

The meltdown spilled across to other banks, as interbank transactions from NAB to its peers also hit the buffers, causing delays in salary and payments processing for some customers.

In a tweet posted Monday, Bank of Queensland observed: "Most NAB payments are now up to date. NAB are still working to fix some issues with duplicate transactions."

In a statement on its Website, NAB says it has now identified and fixed the problem, and that no customers will be left out of pocket as a result of the chaos: "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."

However, the knock-on effects are still being felt across the bank's other systems, with NAB informing customers that "due to high demand, NAB Internet Banking facilities have been running slower than usual today".