The Australian Securities and Investments Commission will investigate allegations that the Commonwealth Bank's life insurance arm is denying heart attack claims by deliberately using an outdated definition buried in the policy.

The allegations were revealed in a joint media investigation by Fairfax Media and Four Corners, which exposed unethical conduct in one of the country's biggest life insurers, which has more than 4 million policyholders and collects $2.5 billion in premiums every year.