Westpac suffered an online banking outage on Thursday morning which rendered the service unavailable across desktop, mobile and tablet.
The service was unavailable for all Westpac online banking customers from around 10am until 7pm.
A Westpac spokesperson declined to comment on the cause but said the bank was investigating and working on a fix.
Westpac is preparing for the introduction of a new online and mobile banking platform, as part of its four-year "strategic investment priorities" program. It is currently three years into the project.
The new system will be underpinned by Fiserv technology and is scheduled for release around late 2013 or early 2014. Westpac signed the deal for the system in July 2010.
The bank revealed in May it had delivered the first two of six releases for the new online platform during the first six months of 2013.
At that time the bank had 3.5 million online customers, 40 percent of those transacting via mobile.
Update: Online and mobile banking went down again Friday morning. Click through for details.