A Telstra outage which caused issues for shoppers and bank customers around the country is been blamed on an "unusually large volume of traffic" in NSW.

The nation’s four big banks and major supermarkets were affected by the outage, which inhibited some EFTPOS terminals and access to ATMs on Thursday. 

Telstra apologised for the outage and says it worked as quickly as possible to restore the disruption to its data services.