Optus customers' private information could be compromised after a cyber attack hit the phone and internet provider.
Millions of customers' names, dates of birth, phone numbers, email addresses and addresses could have been accessed in the attack, Optus has confirmed.
Payment details and account passwords have not been compromised.
Optus says it is working with the Australian Cyber Security Centre to limit any risk to current and former customers.
The Australian Federal Police, the Office of the Australian Information Regulator and other key regulators have also been notified.
"As soon as we knew, we took action to block the attack and began an immediate investigation," Optus chief executive Kelly Bayer Rosmarin said in a statement on Thursday.
"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.
"We are very sorry and understand customers will be concerned. Please be assured that we are working hard ... to help safeguard our customers as much as possible."
