Why would you do this?!

Qatar Airways to multiple Oz cities is 180,000 Avios plus £600.

BA peak date to Sydney is 300,000 Avios plus nearly £1,000.

BA with a GPR to Sydney would be 600,000 Avios plus nearly £1,000.

Even the Qatar option isn’t a great deal if you can get a cash fare under £2k which pre covid was usually possible from somewhere in Europe.