Ahahahah no. The K-T impact was orders of magnitude less energy than you'd need to actually erase Australia as a continent. First off, sinking continental crust is impossible because it's less dense than the mantle it's floating on, so to remove it you'd need to either reduce all of the continent above sea level to orbital ejecta, or melt the entire continental mass and sufficient surrounding mantle material to destroy the continent.Way less than "Meteor that kills dinosaurs" that's suposed to be one of the Antagonists BIG weapons.
Assuming the first since it's far more feasible (any energetic event/impact that could melt a continent can already reduce it to orbital debris several times over) you'd need an impact crater with a radius more than 20 times the K-T impact to destroy australia. Width of Chicxulub crater is 110 miles, width of australia is close to 2500 miles.