A fair way would be to impose a total balance cap. Once you have reached that cap, you can’t add anything more to super. Could be a $3m cap, or something even a little lower. But whatever sits below that cap is allowed to grow beyond the $3m through investment returns (but not contributions), and treated in the normal way.
You might even combine that with a provision where balances currently in excess of, say, $4m need to be reduced to $4m, with the excess withdrawn tax free irrespective of age (for investing outside of super, or personal expenditure, whatever).
Assets transferred would assume a cost base for CGT purposes of the value at the date of transfer (marked to market, where there is a market, such as shares, or an independent valuation for the likes of real estate or art).
