Millions of customers ask their telecommunications providers for a silent or unlisted number so their phone details don't appear in directories, often due to concerns for their privacy or safety.
But the Australian Communications and Media Authority found Telstra had disclosed the details of more than 140,000 customers with unlisted numbers between 2021 and 2022.
More than 24,000 silent numbers were published with corresponding customer names and addresses in the White Pages directory.
Meanwhile, 139,402 unlisted numbers were published with customer details in its directory assistance database, which is used by Telstra operators when answering calls from customers.
Customers had the right to expect their information would be protected, the authority's consumer lead Samantha Yorke said.
"While we are not aware of any harm to people as a result of these breaches, Telstra failing to safeguard customer information, putting people's privacy and safety at risk, is a serious matter," she said.
Systems issues and process failures at Telstra resulted in the release of the unlisted numbers.
Telstra reported this to the authority, which found the company breached the conditions of its carrier licence on more than 163,000 occasions.
The watchdog has required Telstra to reconcile its customer data within the White Pages and its own database every six months, implement a training program for staff and have its systems and compliance procedures independently audited.
Failing this, it could take Telstra to court where the company would face millions of dollars in penalties.
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