"We are impacted by an issue with one of our telecommunication providers that are affecting some of our services. We apologise to all of our customers who are inconvenienced by this," a CBA spokesman said in a statement to AAP.
"The cause of the problem is under urgent investigation and we are working to have full services restored as a matter of priority."
Customers complained to CBA via social media that their EFTPOS machines were displaying error messages.
The same issue hit St George Bank and Westpac.
"We're aware some branches, ATMs and services in Sydney metro are non-operational due to a broader network failure in the region," St George Bank said in a tweet.
Westpac said that some branches, ATMs and services in Sydney were non-operational due to a broader network failure in the region.
The outage has also wreaked havoc at Caltex service stations and Woolworths stores.
"We're aware of a Telstra network issue impacting electronic payments in our stores," the supermarket giant tweeted.
"Stores are continuing to trade as our teams are still able to process payments manually at this time.
"We're working closely with Telstra to rectify the issue as quickly as possible."