The company said the release of the names, numbers and addresses of some unlisted customers was not the result of any malicious cyber attack and was a mistake.
"For the customers impacted we understand this is an unacceptable breach of your trust," Telstra executive Michael Ackland said in a post released online.
"We're sorry it occurred, and we know we have let you down."
Telstra blamed a "misalignment of databases" and was working to pull the data from off the internet.
Impacted customers are being contacted and offered free services to combat identity theft.
"We are conducting an internal investigation to better understand how it happened and to protect against it happening again," Mr Ackland said.
The mishandling of customer data comes in the wake of data breaches at telco Optus and health insurer Medibank, where poor security processes allowed hackers to steal thousands of customers' data.
Home Affairs Minister Clare O'Neil has previously blasted companies for poorly protecting customer data, as the government considers tougher laws to crack down on the handling of sensitive information and hackers.
The incidents with Optus and Medibank raised questions about why companies were retaining sensitive data years after they no longer had a use for it.
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