Australia's second-largest broker ANZ E*Trade was brought offline during the Christmas and New Year break after it was hit with a distributed denial of service attack.
The outage reported by The Australian Financial Review cut access to the online broker service for ANZ customers in the US, Britain, the UAE and across Asia.
The offshore attack overwhelmed ANZ with troves of spam emails sent during 19 and 20 December.
Local customers had intermittent access during the attack, and overseas clients were cut off from the service for up to a fortnight.
IT staff had cut access by foreign addresses and began systematically re-activating access as part of its mitigation strategy.
The bank said security of its services was not compromised.