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.

Another St George customer said internet banking for them had been offline since about 11am AEDT on Sunday, meaning the outage had been ongoing for more than 24 hours as of 11am Monday AEDT.

St George Bank first notified customers of the outage via Facebook at 5.13pm AEDT on Sunday, although many customers said they were unable to access their funds before then.

"The problem has arisen following a regular upgrade to the bank's computer systems over the weekend," St George's website states when customers attempt to log in to internet banking.

"We are currently working to restore banking services as quickly as possible and apologise to customers for the inconvenience that this incident has caused."

Bank of Melbourne and BankSA issued a similar statement on their Facebook pages. The scheduled outage was meant to last from 11.30pm AEST on Saturday until 5am AEDT on Sunday.

While St George Bank said on its website that customers could still access funds using ATMs and eftpos, many customers complained they were unable to transfer funds from their savings accounts to an account accessible using their bank cards.

St George Bank's Facebook page was inundated with complaints; some customers pleaded with the bank to open branches on the public holiday so that they could access their funds.

"Went out for lunch and couldn't transfer money to pay was extremely embarrassing!" St George customer Jules Lewis wrote on the bank's Facebook page on Sunday afternoon.

"All plans now cancelled for this evening as we have no access to funds without internet or phone banking....cheers St George."

Another customer, Chris Garrett, wrote: "Thanks St George, for the ruined long weekend. Thank you for confining my family to the house as our holiday plans require us to access our money... This is an absolute joke.

"You will be losing a record amount of customers come Tuesday."

Another, Brianne Lowe, wrote: "This is absolutely ridiculous!! It's a public holiday and we can't go anywhere or do anything! I can't put petrol in my car, can't buy nappies and I cannot do my food shopping so I have no idea what we will be eating for dinner.

"Hurry up and get it fixed. Your IT guys need a kick up the ass."

A Westpac spokeswoman said it was working to restore services "as quickly as possible" and apologised to customers for the inconvenience that the incident had caused.

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.

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