Millions of customers ask their telecommunications providers for a silent or unlisted number so their details don't appear in directories, often due to concerns for their privacy or safety.
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.
A Telstra spokesperson said the issue was picked up in 2022 and immediately reported to the regulator, with customers made aware and "corrective action" taken.
This included offering free identity theft support.
"Since it occurred, we have significantly upgraded our systems through major software and technology improvements and we conduct regular sweeps to pick up any potential misalignments," the spokesperson said on Wednesday.
The details of more than 140,000 customers with unlisted numbers were disclosed. (Joel Carrett/AAP PHOTOS)
The authority's investigation found 24,000 silent numbers were published with corresponding customer names and addresses in the online White Pages directory.
Meanwhile, 139,402 unlisted numbers were published with customer details in its directory assistance database, used by Telstra operators when answering calls from customers.
People 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, the investigation found.
The watchdog 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.
If Telstra breaches the direction, the authority could take it to court where the company would face millions of dollars in penalties.
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