stop losses and a bit of panic, page-24

OK, CGT is less if you hold a stock for more than 12 months. Where a number of parcels in a particular stock are bought and sold over a period--how does ATO know which bits of your parcels were bought more than 12 months ago? Is it done on the quantities: i.e. if 100,000 were bought more than 12 months ago then only the profit on that number--when they're sold--can be climaed with the reduced tax on profits?