A number of credit card numbers were cancelled by the Commonwealth Bank and Westpac in fear that they might have been compromised and opened to fraudulent activities.

According to ABC news, the Commonwealth Bank detected the potential security breach through its continuous monitoring process. In response to this, the bank immediately cancelled all probable affected credit card numbers.

Furthermore, the cards were possibly compromised through an Australian merchant acquired by another bank, explained News.com.au.

CBA immediately warned MasterCard and Visa of the potential security problem.

"As the Commonwealth Bank takes protection of customer data very seriously, we have moved immediately to protect the accounts of our customers," CBA told ABC news.

As part of their security measures, Commonwealth Bank contacted all affected customers via SMS, email and letters “to advise them their cards have been cancelled and will be reissued as a matter of priority.”

Any fees incurred by the customers due to the card cancellations will be fully reimbursed, CBA declared.

Up to this point, whether the issue is under police investigation is not confirmed yet.