Telstra Tribe users mistakenly gained access to other customers' accounts this month due to a server glitch that randomly linked them to different mobile identities.
The Tribe service allows Telstra's NextG customers to post to and receive notifications from various social networking accounts for free.
"Once you've added Facebook, Twitter and MySpace to Tribe, there's no need to sign into each one separately anymore," according to its website.
The service was taken offline for two days earlier this month after the telco discovered what its head of social media, Kristen Boschma, described as "a fault in the security".
A Telstra spokesman explained that a server associated with the Tribe service suffered a glitch that affected less than 3,000 Tribe accounts.
Some customers were mistakenly sent Tribe SMS alerts that allowed them to access another user's Tribe account.
"A network server glitch resulted in customers being intermittently and randomly associated with a different mobile identity," he told iTnews.
"This meant that some customers were able to access another customer's Tribe account if they visited the Tribe site on their mobiles ... from where it was possible to post status updates/tweets, upload photos or reply to messages."
In a blog post, Boschma apologised for any inconvenience, noting that it was "unacceptable that a customer's privacy might be breached". The post was referenced in a Sydney Morning Herald report yesterday evening.
Telstra is attempting to call all affected customers to explain and apologise.