And thinking on it, one of the stronger candidates for who sold...

And thinking on it, one of the stronger candidates for who sold is tax loss selling. Perhaps they have been talking to their accountant who has confirmed they will have a tax bill if they don't sell shares in one of their loss making holdings. Perhaps they bought the 100k at a much higher price meaning the sale could potentially provide a tax shield larger than the value of the shares sold. If the goal is to get losses to avoid a tax bill that would occur without the sale, there's no point in waiting to execute the sale at a slightly better price. Additionally, if they still like TG6 (they were still holding to 12c), the sooner they sell the sooner they can rebuy without the taxman thinking its a washing trade.

This is some ATO commentary on the subject but anyone would be a fool to think it didn't happen just because of some ATO warnings. Individuals are typically just not stupid enough to do the trades on the last few days of the tax year (or to rebuy within a few days of the sale). Sorry I don't know if there are any safe harbour wash rules, or if so, what they are. I suspect its more a case of needing a reason for selling and then fairly soon after that a different reason needs to be found to justify the repurchase making the tax benefits an incidental benefit that just "happens" to occur. BTW - Clearly this is not tax advice.