The National Australia Bank payments failure these last few days had a cascade effect throughout the sector, with several other banks issuing alerts to customers that some transactions had been delayed as a result of the NAB systems collapse.Classed as a Severity 1 failure - a computer problem that has no obvious workaround - the failure demonstrates the extent to which banks, indeed the entire banking network, as no bank operates in a vacuum, is now reliant on the robustness of their core banking systems and processes. NAB's original problems are understood to have stemmed from a failure of just one element of the legacy batch-processing system that is supposed to process payments overnight. Only on Wednesday night it didn't.Follow on problems are evident from the Thursday failure, and the duplicate and missing transactions.The very public failure is bound to place additional pressure on Christine Bartlett who runs the bank's Nextgen project, which is intended to replace legacy core systems and create a banking platform for the future, using Oracle's iFlex software.  Originally slated as a three to five year project, the bank has revealed little about its progress to date.While replacing legacy batch-processing systems with fresh real-time banking platforms is seen as a first step in delivering greater reliability, it's no guarantee that system crashes won't happen, at least occasionally.Even the most modern computer systems can fail - witness the public relations disaster Virgin Blue endured in September when its information system, provided by Accenture's Navitaire division, collapsed after a hardware failure, leaving thousands of passengers delayed or stranded.Speaking at the AIIA's financial services seminar last Friday, Sam Plowman who is in charge of all direct bank operations at the NAB, including online and UBank, said that during the last 12 months the NAB had not necessarily led the market in terms of innovation, but had been a fast follower. The bank's website was now attracting 170 million visitors a year, reflecting 15-20 per cent year on year growth.  It was also dealing with 100 million online transactions a year, worth $250 billion. The growing popularity of online banking also leaves banks particularly vulnerable to outages or security lapses. Sean Kopelke, a security specialist with Symantec, speaking at an electronic crime seminar last week warned that banks which were perceived to have security lapses online could find their entire clicks-not-bricks business model under assault as customers chose to retreat to the perceived safety of a physical branch. "What would happen if 120,000 people went into the branch rather than risk online banking?" After the weekend it has just endured, NAB just might be able to answer that question.