Information communications systems at Westpac Australia were down this afternoon, affecting ATMs, branches, and Internet banking countrywide.
The unexpected outage has affected all Westpac branches, with ATMs across Sydney's CBD also currently affected by the outage.
No estimated downtime was given by branch staff in the Sydney CBD.
Westpac has not been able to comment on the situation.
Last October, Westpac's customers also experienced severe disruption when "multiple hardware failures" hit the bank's IT systems.