The bank posted on social media about 9pm on Monday that its customers were “experiencing issues” reaching their accounts.
Users of Westpac’s mobile app and website reported being able to access their accounts again about 5am on Tuesday.
Westpac apologised for the overnight outage, thanking customers for their patience.
It said shortly before midnight that its mobile and online banking services were unavailable “following a routine technology update earlier this evening”.
Some customers posted on X during the outage that they could not see their accounts after logging in to the bank’s mobile app.
Users trying to sign in on the bank’s website were met with the message “Westpac Online Banking is currently unavailable”.
According to outage website DownDetector, more than 10,000 people reported problems accessing their accounts on Monday night.