Ann: Boral to return $3 billion to shareholders, page-28

Each shareholder has their own cost to record and maintain for CGT. Nothing to do with share registries who do not know each persons cost.
So do not expect them to do this calculation for you.
Each shareholder needs to make their own adjustments each shareholding including for capital returns, bonus issues, splits, demergers, consolidations etc. etc.
Recommend you keep good records of all transactions re your shares. If not already, use an accountant to help with this, but have your records in good order to save him time.