A cyber crime syndicate has hacked and scrambled the medical files of about 15,000 patients from a specialist cardiology unit at Cabrini Hospital and demanded a ransom.

The attack is now the subject of a joint investigation by Commonwealth security agencies.

Melbourne Heart Group, which is based at the private hospital in Malvern, has been unable to access some patient files for more than three weeks, after the malware attack crippled its server and corrupted data.

The malware used to penetrate the unit's security network is believed to be from North Korea or Russia, while the origin of the criminals behind the attack has not been revealed.

The online gang responsible for the data breach demanded a ransom be paid in cryptocurrency before a password would be provided to break the encryption.