After a threshold. You don't want to tax some poor pensioner who has lived in the same 2-bedroom cottage in Carlton for 60 years, simply because that location is now worth $2Million.
And now it's complicated. How do you find that threshold?
If it's per-person, you'll find rich people divvying up their portfolios to family members to distribute this tax benefit.
If you grandfather it in so it only affects future property purchases, you disincentivise retirees from downsizing to a smaller place, freeing up some 4-bedroom house.
Taxation policies are hard.
Also interesting that Australia's population is now 28 Million. Which is a lot, but still doesn't put us in the top-50 nations by population.