It was a "Block Special Crossing", an S3 is for equities and is...

It was a "Block Special Crossing", an S3 is for equities and is for values of $200k - $1m.

It could be a trade negotiated between two parties or it could be that someone is getting their holdings switched between accounts in readiness for the Tax man, maybe doing it at a lower price and transferring it to another account will help pay less tax, especially if the ATO considers one a trader and they have to submit holdings by financial years end.