The big four bank was besieged by multiple failures to technology systems across its network on Monday.

The Mobile Banking app, Desktop Internet Banking, NAB merchant EFTPOS terminals and NAB Trade were initially affected.

At 3pm it said "some services are recovering".

"NAB Trade, NAB Connect & NAB Merchant terminals are coming back online," it tweeted.

"Our teams continue to work to fix the outstanding issues with Desktop IB and the NAB Mobile Banking App."

The bank addressed the problems on Twitter about 11am.

"We're currently experiencing login errors with the NAB Mobile Banking app and Desktop Internet Banking. We're working to fix this as soon as possible. We're sorry for the inconvenience," the NAB tweeted to more than 70,000 followers..

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An hour later it tweeted: "NAB merchant terminals and NAB Trade are also unavailable but ATMs, NAB cards and Apple Pay are still available. We're really sorry for the impact this is having. We know this is not how anyone wants to start their Monday and we're working to fix it as soon as possible."

There have been no updates since midday.

Understandably, the breakdown prompted a series of unhappy responses.