all opes client entitled for refund of all tax, page-19

First off there is next to zero chance of the opening an account with Opes being treated as a transfer at nil cost

second if assuming the ATO takes the same view on the contracts as ASIC and treat the transfer of shares to Opes as a sale, there would have been no liability what so ever for Opes to inform the clients (the seller) of their CGT liabilities at the time.

You sell a property to me it is not up to me the purchaser to inform you of your potential tax liabilities in selling your property. It is your responsibility to know your tax and potential tax liabilities