Each batch purchased is treated independently. So if you bought 100 on 1/12/2019 and 200 on 1/6/2020 and sold all 300, the 100 bought on 1/12/2019 qualifies for the discount, the other 200 don't.
If you were to sell just 80, it is up to you whether those sold came from the 1/12/2019 batch (thus qualifying for the discount) or from the 1/6/2020 batch (not qualifying). You could also deem that some came from the first batch and the rest from the other batch (say 20 from 1/12/2019 batch and 60 from the 1/6/2020 batch) if that if beneficial to you from a tax point of view. You will need to keep good records for such split transactions to show that was what you did and was not a retrospective change to defraud the ATO.
