Optus, the Australian unit of Singapore Telecommunications, said on Thursday it was investigating a data breach that could have exposed personal information including home addresses, passport numbers and phone numbers of customers.

Local media outlets reported that up to 9 million customers have been affected by the cyber attack.

Reuters could not personally verify the number as Optus did not immediately respond to a request for a comment.

Optus said it immediately shut down the attack after discovering it, and that payment details and account passwords had not been compromised.

The company's services, including mobile and home internet, are not affected, and messages and voice calls have not been compromised, it said.

"Optus services remain safe to use and operate as per normal," the company said in a statement, adding that it has notified the Australian Federal police and other key regulators regarding the breach.

  - Reuters