You ask this like we don't know the answer. We had the draft in living memory.
We need a proclamation of a call to war before parliament 90 days before it can be activated. The government needs parliamentary approval from both houses. All Australian residents 18-60 can be called up (not only men in the 21st century) but the government can call up a subset of this cohort ('only men', 'only women with red hair', 'only residents of Bankstown' or whatever 'class' they wish).
Then everyone loses their shit, we storm our politicians and threaten them with being the first to war or something if they even think of voting this in. It would be so wildly unpopular and I can't see any government passing it.
It's not something the PM or Governor General can unilaterally do. Hooray for checks on politicians in Australia.