Tens of thousands of customers of Westpac-owned St George Bank, the Bank of Melbourne and BankSA were reeling on Monday following an outage of online, mobile, and telephone banking at the banks.

To add further salt to customers' wounds, the banks' branches were closed on Monday in NSW, Queensland, South Australia and the ACT due to the Labour Day public holiday.

In a statement to Fairfax Media, Westpac said closed branches would not be opening.

It's unclear exactly when the outage began, but one St George customer told Fairfax Media she had been unable to access her funds since Saturday.