The national head of class actions at Maurice Blackburn, Andrew Watson, said this admission of board knowledge about the reporting issues in 2015 would form the basis of the claim for non-disclosure.

"Ordinarily in these cases we have to argue that the company ought to have known, but in this matter we know the board did in fact know and for whatever reason did not disclose," Mr Watson said.

CBA has around 800,000 retail shareholders but under class action law, only those who purchased shares and held some of them during the period of alleged non-disclosure will be able to participate in the action.

Maurice Blackburn and IMF are taking registrations from shareholders interested in joining the action, although it is likely the case will be structured as an 'open class', meaning any purchasers in the relevant period will be entitled to recover losses.

CBA told the ASX on Wednesday afternoon it had not been served with any proceedings and "will keep the market informed of developments" regarding the case.

CBA's share price fell from an intra-day high of $84.69 on Thursday, August 3 - the day the AUSTRAC action was filed in the Federal Court - to an opening price of $80.11 on Monday, August 7, a fall of $4.58 or 5.4 per cent on trading volumes more than 2.5 times greater than the preceding three month average, IMF said. On the Friday after the AUSTRAC proceedings had been filed the previous day, CBA shares fell 3.8 per cent, which shaved close to $5 billion from the bank's market capitalisation.

Mr Watson said the firm's investigations and analysis show the drop was statistically significant because it was in the top one per cent of price movements CBA experienced in the past five years, "making it apparent that the news was of material significance to shareholders".

CBA chief executive officer Ian Narev has argued disclosure of the claims back in 2015 was not necessary because CBA constantly receives regulatory requests and the market would be overwhelmed if each of these was disclosed.


"All major financial institutions have a raft of notices from very active regulators, as you would hope they would, and you simply can't make disclosure every time you get notices from regulators or you would be making disclosures every day," he said after delivering CBA's full year profit on August 9.

"Our view would be these things didn't come anywhere near it in the form they came at the time," Mr Narev said.

While the class action could use information that come out in AUSTRAC's Federal Court proceedings, Mr Watson said that it was not necessary to wait for the AUSTRAC proceedings to be completed as the shareholder case will be able to gather evidence through legal discovery procedures.

Other law firms are also understood to be considering CBA class actions.