Ann: Boral to return $3 billion to shareholders, page-20

Acknowledging that this is a tax-question, if I have two sets of Boral shares - one bought at 2.20 and one bought at 4.85 - with an average purchase price of 3.20. How do I go about this?

I usually do CGT calculations based on the purchase price of each lot but if I do that in this instance, I may need to pay CGT tax in advance for the lower price lot which I would like to avoid. Does anyone know whether I can use the average price as my cost base instead?