Telstra is being investigated by both the communications and privacy watchdogs after it sent out 220,000 letters that contained account information belonging to other customers.

The letters, which contained the name, phone number and telephone plan of customers other than the recipients, explained upcoming fixed line price changes.

Telstra blamed the privacy snafu on a "mail-merge error", which caused the 220,000 letters to be sent to the wrong addresses. Twenty-three thousand, five hundred of the letters involved customers with silent lines.

In a joint announcement today, the Australian Communications and Media Authority and the Office of the Privacy Commissioner said they were looking into the circumstances surrounding the botched mail-out.