The statement of claim alleges Optus breached the privacy of its customers as well as telecommunications law and consumer laws.

An estimated 10 million people were thought to have been affected by the massive data breach in September.

Slater and Gordon said the company failed to protect or take reasonable steps to protect customers’ personal information from unauthorised access or disclosure, failed to destroy or de-identify formers customers’ personal information and failed to ensure that only those who had a legitimate reason for having access to the information could access it.

Slater and Gordon said Optus had been accused in the class action of breaching contractual obligations to customers along with its duty of care to ensure that customers did not suffer harm arising from the unauthorised access or disclosure of personal information.

The firm’s class actions practice group leader Ben Hardwick said the breach was extremely serious both in the number of people affected and the nature of the information that was potentially disclosed.

“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,” 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 measures in place to prevent that.”

He said the data breach had also potentially jeopardised the safety of a large number of vulnerable customers, such as victims of domestic violence.

Among Slater and Gordon’s registered Optus customers was a stalking victim who now has to take extreme measures to protect her privacy. There was also a woman who was too scared to answer the telephone after she received a significant increase in scam calls and a retired police officer concerned that his home address may have been shared with criminals he helped prosecute.

The case’s lead applicant, who wanted to remain anonymous, said he felt vulnerable, exposed and worried by the breach.

“Not knowing what still might happen as a result of having my information accessed and by whom haunts me,” he said.

“It feels like it’s only a matter of time before I get scammed or defrauded, which is a constant worry that I didn’t have before I was let down by Optus.”