Glitch-prone National Australia Bank and its UBank subsidiary suffered a five-hour outage Wednesday after a network failure at the bank's ageing East Melbourne data centre.
The overnight breakdown hit NAB's Internet and telephone banking services and wiped out online subsidiary UBank.
NAB has experienced a series of computer collapses over the past two years as it discards 100 legacy applications, decommissions data centres and transitions to a new core banking platform.
The bank is barely one-third of the way through the multi-year programme, which last week saw the migration of 300,000 UBank customers to the Oracle-built i-Flex system.
The fragility of the nation's banking systems has led the Australian central bank to promise a crackdown on outages and the introduction of a formal reporting system for major IT incidents.