A class action against Optus involving 100,000 customers impacted by last year’s data hack will be lodged in the Federal Court on Friday.

It could be one of the biggest – and most valuable – class action cases in Australian legal history, according to experts.

The ABC reports that lawyers Slater and Gordon have confirmed the major development.

The firm cited the “anxiety, distress and danger” that impacted customers whose details were stolen, and some of them exposed.

Optus says it will vigorously defend any such proceedings.

Last year Slater and Gordon described the theft of customers’ personal information as “potentially the most serious privacy breach in Australian history”.

The data of nearly 10 million former and current customers was stolen by criminal hackers, forcing the telco into damage control.

The personal data included names, addresses, emails, phone numbers, license numbers and passport numbers.

The hacker posted the details of 10,200 of the customers on the dark web when Optus refused to pay a $1 million ransom.

The data breach was the first of a wave of hacks last September and October that hit major Australian corporations including Medibank Private, EnergyAustralia and Woolworths.

The hack sparked a criminal investigation from the Australian Federal Police, working with the Australian Signals Directorate, to determine the culprits and how the attack occurred.

Slater and Gordon’s Ben Hardwick told the ABC Optus had failed in its duty of care to protect customers.

“We have people who work in frontline occupations — police officers — very, very concerned that that criminals will find out where they live,” Mr Hardwick said.

“So the the release of this information causes people great anxiety. It causes them great concern.

“And, so, this class action is about seeking redress for the potentially millions of Australians who have been affected by this data breach.”

In March, Optus CEO Kelly Bayer Rosmarin said no customer had suffered financial harm as a result of the hack.

-with AAP