Telstra agreed more than a decade ago to store huge volumes of electronic communications it carried between Asia and the US for potential surveillance by US intelligence agencies, in a secret agreement with the FBI and the US Department of Justice.

On Friday, Telstra was refusing to say whether it had similar data retention agreements with other nations' intelligence agencies, including those in Australia.

Telstra: Storing data for potential surveillance by US intelligence. Credit:James Davies

Australia's other major telco, Optus, declined to say whether it stored data for potential surveillance by US, or Australian, authorities.

Under the previously secret US agreement, Telstra has been sending all communications involving a US point of contact through a secure storage facility on US soil that is staffed exclusively by US citizens carrying a top-level security clearance. The data includes the content of emails, online messages and phone calls.