Medibank has confirmed almost 500,000 health claims were accessed and the personal details of former and current customers were exposed when an unnamed group hacked into its system weeks ago.

Around midnight, a ransomware group posted to its darkweb blog that “data will be publish (sic) in 24 hours”.

“P.S. I recommend to sell (sic) medibank stocks.”

The post did not include data samples to back up its threat.

“This is horrendous, but not unsurprising if you look at ransomware like a business,” cyber security expert Troy Hunt said on Twitter on Tuesday.

“If they *don’t* dump the data publicly, what message does that send to future ‘customers’?”

Medibank chief executive David Koczkar on Monday said paying a ransom could make Australia “a bigger target” for data thefts by giving criminals an incentive.

“Based on the extensive advice we have received from cybercrime experts we believe there is only a limited chance paying a ransom would ensure the return of our customers’ data and prevent it from being published,” he said.

Home Affairs Minister Clare O’Neil said Medibank’s decision not to pay a ransom to cyber criminals was in line with government advice.

Opposition home affairs spokeswoman Karen Andrews said the government had dropped the ball on cyber security and called for support of a proposal to introduce a standalone offence for cyber extortion.

Meanwhile, two law firms, including one behind a successful case involving an Ambulance NSW data breach, say they believe Medibank betrayed customers and breached the Privacy Act by not stopping the hack.

“Medibank has a duty to keep this kind of information confidential,” Bannister Law and Centennial Law said in a statement late on Monday.

“This latest data breach exposes the lack of safeguards in place to prevent such personal and private information being released to wrongdoers and Medibank & ahm have failed policyholders in these circumstances.”

The law firms will investigate the terms of the contracts the medical insurance provided to customers and whether damages are appropriate.

No case has been filed with a court.

Affected customers can register on the law firms’ websites.

The hacker accessed the health claims of about 160,000 Medibank customers, about 300,000 claims from offshoot ahm customers, and about 20,000 international customers.

Names, dates of birth, address, phone numbers and email addresses were also accessed, raising concerns about future identity fraud.

No credit card or banking details were accessed.