Tens of thousands of customers of the three Westpac-owned banks were reeling on Monday after an outage of online, mobile, and telephone banking.

Banking services were progressively restored on Monday night, but some customers still reported being unable to access their mobile banking on Tuesday.

The Bank of Melbourne and St George advised customers still experiencing problems with mobile banking to uninstall and then reinstall the banking app on their mobile phone and then try to log in again.

"Quit/kill your app and it will clear the cache, which should allow you to log in," the Bank of Melbourne told one customer on Facebook.

The bank has promised to reimburse any fees or interest customers may have incurred from payments that were not on time as a result of the outage.

"We are sorry for the inconvenience this outage caused for some of our customers, and thank you again for your patience," Westpac said in a statement.

The problem was a result of a scheduled system outage that went wrong in the early hours of Sunday morning after Westpac's mainframe computer became "corrupt".

The outage was meant to last from 11.30pm on Saturday night until 5am on Sunday, but lasted well into Monday.

It caused even greater headaches for customers in NSW, Queensland, South Australia and the ACT due to the Labour Day public holiday in those states.

Customer transactions undertaken at the weekend should be processed Monday night or Tuesday morning, Westpac said.

The outage follows a widespread technology outage at the Commonwealth Bank that affected credit cards, net banking, mobile banking, phone banking ATM, Bpay and eftpos services on September 11.

A further three-hour long outage in June affected Commonwealth Banks' eftpos machines and customers' access to some features in Netbank, its online banking system.