Medibank CEO apologises for 'horrendous crime' as customers remain in the dark about extent of hack

Nearly four million Australians remain in the dark about whether their sensitive medical information is in the hands of hackers a week after Medibank was attacked.

The health insurer was the victim of a cyber breach and while it initially reassured customers their data was safe - this is no longer the case.

"I am very sorry this has happened.. This is a horrendous crime," Medibank CEO David Koczkar told 9News.

READ MORE: Man jailed for life over 'gruesome' murder after victim's body fed into woodchipper

"I feel devastated, I feel disappointed and I feel for all of our customers, who will feel anxious, concerned and let down."

Koczkar said after the hack last week Medibank did not "see any evidence that any customer data had been removed from our systems".

However since then, the hackers have shown proof they have the personal data of at least 100 customers.

This data includes names, addresses, dates of birth, email addresses - and most concerningly - private and potentially sensitive medical information.

The private health insurer has close to four million customers and today Medibank admitted the full extent of the hack isn't yet known.

READ MORE: Why Aussie waters have become ideal hunting ground for great whites

Medibank won't say if it's responded to the hackers or whether Australia's largest private health insurer would pay a ransom to protect its customers.

"You would appreciate, given this is a matter of high sensitivity we aren't going to comment on that," Koczkar said.

Medibank said its cyber security repels up to 250 million attempts to breach its database each month and another five million emails each month trying to compromise its system.

MONEY MATTERS: Can hairdressers charge twice for the same haircut?

Home Affairs Minister Claire O'Neil has warned "the damage could be irreparable".

"We are behind the eight-ball here and I think that's clear from what we've seen over the last month."

Medibank has requested the trading halt on its shares be extended into next week due to the uncertainty.