Australia is about 7 and a half million square kilometres, that is roughly the size of the Siberian traps. And the answer to your question is that nobody really knows but when the Siberian traps were formed it is estimated 90% of all species went extinct. The reason the answer is unknown is because events of this magnitude cause an entire chain of events that might or might not repeat at a later time. Specifically the response of the ecology would be quite different depending on which stage of evolution the event happens. Likewise traps may be triggered by asteroid impacts, which themselves are extinction level events.
It should be noted that the ecosystem needed 15 million years to recover.
Supervolcanos are basically smaller cousins of the above (one massive eruption instead of a series of eruptions lasting millions of years) and too small to create Australia. Their consequences (outside the immediate area) are mostly limited to global cooling. But the cooling may last from a one year without summer to a thousand year small ice age killing most of human population.