Thousands of Colonial First State customers are mounting a Federal Court class action against the CBA-owned wealth manager for allegedly dumping their superannuation money in uncompetitive deposit products marketed by the bank.Slater & Gordon is mounting the legal proceedings on behalf of the customers who claim trustees of Colonial-managed super funds failed to invest in alternative cash products that delivered higher returns."We will allege that by dumping members' super with its parent bank, the CBA, Colonial First State failed to obtain the most competitive interest rate available for its members invested in cash-only investment options and balanced options where there is a cash component," said Slater and Gordon's head of class actions, Ben Hardwick."The class action will allege there is no excuse for Colonial First State to have accepted such a low rate from CBA when it could have easily obtained a higher rate - either from the CBA or from any other bank."In a statement filed to ASX on Wednesday afternoon, CBA said the bank and its Colonial subsidiary would vigorously defend the court action."Commonwealth Bank of Australia acknowledges that class action proceedings concerning investment in cash and deposit options in certain funds have been filed by Slater and Gordon in the Federal Court of Australia against Colonial First State Investments Limited (CFSIL), a subsidiary of CBA," the bank told the ASX."CBA is also a respondent in the proceedings. "CFSIL and CBA will vigorously defend the proceedings."It is believed that the plaintiffs are seeking compensation of up to A$100 million for the alleged practice, which if proven, would establish that independent trustees of the Colonial funds failed to act in the interests of members.While only several thousand members of Colonial funds have joined the class action, Slater and Gordon believes that "hundreds of thousands of customers" are potentially eligible to get money back from Colonial and the bank.Hardwick said the class action would argue that Colonial placed the interests of its members beneath the interests of the bank."A superannuation fund trustee is obligated to act in the best interests of its members, not its parent company," he said."CBA has been paying some Colonial members rates as low as 1.25 per cent, below even the RBA cash rate at 1.5 per cent. Slater & Gordon estimates that if CBA had paid just an extra 0.5 per cent per annum to a member with $100,000 in a cash investment option, that member would have earned an additional $2,235 over five years.Lead plaintiff, Keith Kayler-Thomson, said he could not believe how blatant the unethical behaviour appeared to be."These are my retirement savings we're talking about," he said."I assumed that a big institution like Colonial could be trusted to act ethically and within the law."Slater and Gordon has invited current and former members of five Colonial funds and one CBA-branded fund to register their interest in the case at a dedicated website known as getyoursuperback.com.The UK's largest litigation funder, Augusta Ventures, is funding the class action.It is the largest investment made by the