IVZ Share Price, page-13554

https://www.ato.gov.au/uploadedFiles/Content/IAI/Downloads/Toolkits/TaxTimeToolkit_Captialgainstaxonsaleofsharesandunits.pdf

You've got a few choices, the most common is first in first out as it is simplest, but, you dont have to use that method if you dont want to.

Not entirely sure what you mean by selling at an average price for the whole lot. If you have purchase the shares on different days, for each parcel, you will need to calculate if the 50% discount applies.

ie.
Bought 100 shares 2 years ago @ $1 each = Cost base = $100
Bought 100 shares 6 months ago @ $2 each = Cost base = $200

Sold 200 shares today @ $3 each = sale proceeds $600

100/200 = 50% (out of your $600 proceeds, 50% applies to first lot & 50% applies to second lot as it is the same # of shares)

$300 - $100 = Capital Gain of $200 on first lot. You get the 50% discount on this as you have held for more than 12months = taxable gain of $100
$300 - $200 = Capital gain of $100 on second lot. No discount as held less than 12months.

If you are not confident with this you really should go seek out advice from a registered tax professional to assist.