The fault is the latest in a series of banking outages to have struck the nation's biggest banks as they rip out legacy technology and migrate to more modern core banking systems.

In a statement, Westpac group executive retail and business banking Rob Coombe says: "Westpac sincerely apologises to all our customers who have been impacted by today's outage. We take systems reliability extremely seriously and are very disappointed by the inconvenience to our customers and will undertake a thorough review."

He says the bank has now revived ATM and Eftpos services and is working to restore full functionality to online banking.

Westpac recently extended an infrastructure outsourcing deal with IBM as part of an effort to mitigate the risks from a major upheaval as the bank implements a five-year IT transformation programme.

Other Australian banks to have suffered embarrassing tech failures in recent months include Commonwealth Bank and NAB, both of whom are undertaking extensive IT upgrade programmes.

In a presentation to analysts this morning NAB CEO Cameron Clyne, said the bank's customers could expects to see the first fruits of its $1 billion IT modernisation programme within the next six months.

He described the recent spate of outages as the result of a 40-year-old infrastructure that the bank is quickly trying to change.

"Outages and things are the result of an aging environment," he said. "We're fixing that, we're definitely on the path to fix it."