Telstra has blamed "an unusually large volume of traffic" in NSW for the national outage that plunged customers and retailers on its network into chaos on Thursday afternoon, as pressure mounts for banks and telcos to improve their systems.

The outage, which impacted ATMs and eftpos machines across the country, was largely resolved by 7pm on Thursday, with a Telstra spokesperson said all services had been restored on Friday.

The Telstra outage on 11 July impacted some of Telstra’s IP network services, including eftpost and ATMs. Credit:Patrick Scala

"We are continuing to investigate the exact cause of this issue but early investigations suggest it was caused by an unusually large volume of traffic across network links in NSW," a spokesperson told the Herald and The Age.

The four big banks, as well as the Bendigo and Adelaide Bank, began to experience disruptions to their services Australia-wide about 2.30pm on Thursday.