Millions of customers' names, dates of birth, phone numbers, email addresses, driver's licence numbers, passport numbers or addresses could have been accessed in the attack, Optus has confirmed.

Payment details and account passwords have not been compromised.

Scamwatch said Optus customers are at risk of identity theft and urged people to take urgent action now.

"Optus customers should take immediate steps to secure all of their accounts, particularly their bank and financial accounts," government watchdog said.

"You should also monitor for unusual activity on your accounts and watch out for contact by scammers."

Steps Optus customers should take to protect personal information:

Scammers may use your personal information to contact you by phone, text or email.

Never click on links or provide personal or financial information to someone who contacts you out of the blue.

If you are concerned that your identity has been compromised or you have been a victim of a scam contact your bank immediately and call IDCARE on 1800 595 160. IDCARE is Australia's national identity and cyber support service, to get expert advice from a specialist identity and cyber security service.

IN OTHER NEWS

Optus CEO Kelly Bayer Rosmarin said on Thursday afternoon she was not aware of "customers having suffered any harm".

"We encourage customers to have heightened awareness across their accounts, including looking out for unusual or fraudulent activity and any notifications which seem odd or suspicious," she said.

Check www.optus.com.au for information and contact Optus via the My Optus App or call 133 937.

Australian Federal Police, the Office of the Australian Information Regulator and other key regulators have also been notified.