(Bloomberg) -- Singapore Telecommunications Ltd. said its Australian unit Optus is investigating a cyberattack that may have led to the unauthorized access of current and former customers’ information.

Customer names, birth dates, phone numbers and email addresses are among the data that may have been exposed, SingTel said in a statement to the Singapore exchange Thursday. The attack may have exposed the addresses and identification-document details of a smaller number of customers, it said.

“As soon as we knew, we took action to block the attack and began an immediate investigation,” Optus CEO Kelly Bayer Rosmarin said in the statement. “While not everyone may be affected and our investigation is not yet complete, we want all of our customers to be aware of what has happened as soon as possible so that they can increase their vigilance.”

The company said it’s working with the Australian Cyber Security Centre to mitigate risks to customers, and it also notified the Australian police, the Office of the Australian Information Commissioner and key regulators. Optus services, including mobile and home internet, weren’t affected and the company says its services remain safe to use. 

Australian markets are closed Thursday for a national public holiday to commemorate Queen Elizabeth II.

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