According to press reports, NAB customers were locked out of Web banking accounts when the system broke down at around 9 am on Monday. The service wasn't restored until late afternoon.

NAB public affairs officer Rebekah Miles told reporters that the bank was still trying to work out what went wrong, but said it did not appear to be connected to any systems upgrades.

The outage comes just two days after NAB rival Bank of Queensland (BoQ) experienced problems with its online banking service following routine IT upgrades.

Some BoQ customers were unable to access accounts after routine upgrades left the bank's online services congested and unable to operate at full capacity at peak times.

BoQ's technical staff worked over the weekend to fix the system. On Monday BoQ corporate affairs and customer relations group manager Paul Turner told local press that the problem had been identified and fixed, although he declined to go into further detail.

Turner said that there is no indication that the BoQ and NAB incidents are linked.

Meanwhile Commonwealth Bank of Australia (CBA) also experienced systems problems last week that affected its CommSee teller system and online business banking application CommBiz.

The glitch, which occured after a routine upgrade of the Oracle platform, has resulted in delays for retail customers getting loan approvals and opening new accounts at branches.