Buy it on arrival in Sydney. You have to march through the inbound duty free section before you get to immigration anyway and for the sake of the extra coupe of A$ per bottle, its worth shopping on arrival. UNLESS of course you’re being asked to bring something special in terms of a whiskey etc as the selection is relatively standard in the spirit line. Don’t buy any wine in Australia duty free. It’s cheaper retail in what will be your local sydney bottle shop!
You are allowed 2.25L of alcohol into Australia per person.
Lugging it from Heathrow is an (expensive) pain.
Oh and Singapore has always had an inbound duty free.