Here's how to figure this out. Earth is a planet. Mars is another planet. So the relationship between them is that they are two examples of the same basic kind of thing.
So then you ask "what is Australia?" You could answer this in two ways. It is either a continent or a country. If you decide to treat it as a continent, then the answer is some other continent like Africa. Or maybe the best one would be Antarctica because it, like Mars, is not populated. At any rate, the analogy is correct if you pick another continent because then Australia and the other continent would be two examples of the same kind of thing just like Earth and Mars.
Earth is to Mars and Australia is to the Amazon Rainforest. This is because earth is healthy and populated where as Mars is desolated, dry and has no living creatures. Therefore Australia was a desert previously, with no inhabitants and the Amazon Rainforest was populated with a multitude of species and plants.