The personal information of up to nine million Optus customers has been compromised after a massive data breach.

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The cyber attack took the personal details of around 2.8 million people, including their passport and licence numbers, email and home addresses, date of birth and phone numbers.

A further seven million had their dates of birth, email addresses and phone numbers stolen.

Tech expert Trevor Long said he believes it’s the biggest breach of an Australian company.

“Every Optus customer has to be on alert.”

He said customers need to be vigilant, check accounts and change passwords.

Press PLAY below to hear him explain more of what we know so far

It’s understood hackers exploited a weakness in Optus’ firewall.

In a statement Optus said it “immediately shut down” the attack once it was aware of the breach and the Australian Federal Police have been notified.

“We are very sorry and understand customers will be concerned,” Optus CEO Kelly Bayer Rosmarin said.

“Please be assured that we are working hard, and engaging with all the relevant authorities and organisations, to help safeguard our customers as much as possible.”

Image: Getty