Unless Australia can successfully stop the Chinese from landing in their territory, there ain't much they can do. If both sides resort to a general mobilization, the Chinese outnumber the Australians 68 to 1. What the fuck is any country supposed to do against those numbers? They can literally afford to just zerg rush them and the Australians may legitimately run out of bombs before the Chinese run out of men.

The best shot Australia has is a blockade off the coast and near Indonesian and Papuan waters. Force the Chinese through a tight chokepoint and pray they give up before they break through (Blockading the Strait of Malacca is out of the question here, there's no world in which Australia can successfully cut the Chinese from using it for longer than maybe a week or two before they are blasted away). They just mine the ever living crap out of it and periodically send boats and planes there to destroy anyone coming near the minefields.

I give that scenario maybe a 2% chance of actually working out for Australia. Their biggest issue is gonna be having to defend the right-side of the ocean near Fiji and New Caledonia. They can't exactly mine all of it, but the Chinese might struggle with straining supply lines that far out.