I understand the CGT discount but what I was asking was let's...

I understand the CGT discount but what I was asking was let's say I have 5 parcels at:

5000 shares bought at 15 cents
5000 shares bought at 16 cents
5000 shares bought at 20 cents
5000 shares bought at 25 cents
5000 shares bought at 30 cents

All were bought more than 12 months ago, and I decide to sell 5000 shares at 35 cents. .

So, the tax payable will vary depending on which parcel I use when tax return is due.

Who decides which parcel is used (for tax calculation) and how is it managed long term - I surely can't just use the 30 cent parcel every time i sell lol