A class action has been launched against Optus following last year's data breach.
Slater and Gordon will represent more than 100,000 participants in the lawsuit.
Lawyers will claim Optus failed in its duty of care to protect customers, and will accuse the telco of "breaching privacy, telecommunication and consumer laws".
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Slater and Gordon class actions practice group leader Ben Hardwick said it was an "extremely serious privacy breach"
"Very real risks were created by the disclosure of this private information that Optus customers had every right to believe was securely protected by their telecommunications and internet provider company," Hardwick said.
"The type of information made accessible put affected customers at a higher risk of being scammed and having their identities stolen, and Optus should have had adequate measures in place to prevent that."
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He said the breach had also potentially jeopardised the safety of vulnerable people such as victims of domestic violence and stalking.
Nearly 10 million Australians had their personal data exposed in the hack, with 2.8 million seriously impacted with passport, drivers licence and Medicare numbers breached.
"Slater and Gordon has advised Optus that it has filed a class action with the Federal Court in relation to the criminal cyber-attack undertaken against Optus in September last year," an Optus spokesperson said.
"As indicated previously, Optus will vigorously defend any such proceedings."
In March this year, Optus CEO Kelly Bayer Rosmarin said no customers had "suffered any financial loss or fallen victim to a crime through misuse of the data".
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