The private medical information of an unknown number of Commonwealth Bank Insurance (CommInsure) customers was made available to employees from unrelated branches of the company, media reported on Monday.

An investigation by the Australian Broadcasting Corporation (ABC) has revealed that the Commonwealth Bank of Australia (CBA) became aware of the mistake in July as they prepared to sell the insurance component of the business for 2.7 billion U.S. dollars.

"We have identified that some internal Group-wide systems also have access to CommInsure linked systems and data," CBA said in a statement.

However, the bank said it's not yet clear how many people were affected and whether the information was actually accessed by staff from the unrelated branches of the company.

According to the CBA, they are investigating the issue and have notified the relevant regulatory bodies.

"We understand that some customers will be concerned about this shared internal access and we are taking steps to ensure access to all sensitive information associated with CommInsure is provided on a need to know basis," a CBA spokeswoman said.