The Commonwealth Bank says some customers remain unable to access services despite earlier tweeting that a technical problem had been resolved.
Several services including ATMs, EFTPOS and internet banking were down for at least four hours on Tuesday, with customers taking to social media to complain.
The bank announced on Twitter at 1346 (AEST) that all services had been restored, but soon followed up with an announcement that some customers remain locked out.
Several customers have said on Twitter that they remain unable to access services.
A spokeswoman could not confirm what had caused the outage.
"We sincerely apologise to any customer who has been inconvenienced with this disruption to services," she said, adding that call centres were swamped and that callers would likely encounter delays.
