Westpac has blamed an overnight server upgrade for an online banking outage that lasted about nine hours yesterday.
The patch caused the page to respond too slowly to load for an estimated three quarters of the bank's retail customers between 7.30am and 4.30pm.
As of 1pm, the cause of the issue had not yet been identified. However, a spokesman said Westpac's support staff had "worked through the day to isolate the problem".
"It's all up and running as per normal," the spokesman told iTnews at 5pm yesterday. "There was an upgrade last night to a server. It did take longer to fix than usual."
The spokesman said the bank's back-up processes kept its business customers and 25 percent of retail customers from experiencing the outage.
"Our back-up systems ensured that it was business as normal for business customers," he said.
Westpac apologised to all affected customers.
Earlier this week, a "critical security change" at the Commonwealth Bank caused otherwise valid MasterCard and Visa transactions to be declined at point-of-sale terminals, and some machines on the bank's nationwide ATM network to malfunction.
The ANZ bank also experienced a "system outage" last Tuesday that left merchants unable to perform EFTPOS and credit card point-of-sale transactions between 11am and 1.30pm.