Westpac has been unable to identify the cause of "intermittent problems" that stopped customers from accessing its internet banking service since 7.30am today.
A spokesman told iTnews that customers had difficulty loading its secure online banking page, which he described as a "degraded service".
The bank's support staff were investigating the cause of the issue, and did not have an estimate of when it would be resolved, he said.
"We are experiencing some slow response times today," the spokesman said. "We are investigating what has occurred."