A class action suit that could exceed A$100 million was filed against Commonwealth Bank of Australia and its superannuation business Colonial First State from law firm Slater and Gordon.

The class action will allege that Colonial First State invested the retirement savings of its members with parent CBA, where it received uncompetitive bank interest rates. The suit will allege that Colonial First State failed to obtain the most competitive interest rate for its members, said Ben Hardwick, Slater and Gordon head of class actions.

CBA has been paying Colonial members rates as low as 1.25%, below the central bank cash rate of 1.5%, Hardwick added.

CBA had in September denied allegations that it engaged in criminal conduct in its dealings with its superannuation fund customers. The country's royal commission had found that banks engaged in "ethically unsound" practices and possibly criminal offenses, such as charging dead customers fees and tardy breach reporting.