You have to pick:

This would produce a global catastrophe. The problem is not so much the carbon dioxide and organic molecules, as it is the enormous quantities of carbonate-containing rock strata. Limestone and dolemite, in particular, form many-kilometer-thick strata over much of Earth's surface. For some reason Google doesn't give me a straight answer to the query "how much limestone is above sea level in Australia" (surely I'm not the first to be searching for this information?) but the answer is probably "lots". Limestone is mainly calcium carbonate, CaCO3. That would turn into CaNaO3, which is chemically nonsensical and therefore completely unstable. I expect it would rapidly reform into CaO, Na2O, Na2O2, O2, and a great deal of heat. If the temperature of the products goes above 675 degrees Centigrade (which seems likely to me given the massive chemical disequilibrium Australia is now in) the sodium peroxide decomposes into more Na2O and releases more O2.

So basically, the ground explodes and releases a whole bunch of blazing hot oxygen. Oxygen is nice to have in moderation but you can have too much of a good thing. We probably won't suffer from oxygen toxicity - you need to be exposed to *high pressure* oxygen for that - but if Earth's atmosphere goes above around 30% oxygen even wet vegetation will burn violently, so wildfires will sweep the planet.

This is not the worst, though. Na2O + H2O -> 2NaOH. Sodium hydroxide is lye, an extremely caustic chemical. Once the red-hot wasteland of Australia cools below the boiling point, rainfall begins creating sodium hydroxide and washing it into the sea. Pretty much all ocean life dies in short order. Many metals are dissolved by sodium hydroxide, so worldwide shipping grinds to a halt as all the ships sink. With all sea life wiped out the world's ecology is thrown even further out of whack, and lots of carbon dioxide is released to join what's being emitted by all the fires going on. Global warming cranks up big time.