The 'Land Down Under' is highly improbable - a great Australian fallacy exposed!
No, Australia most certainly does not exist. Not only is it too large to be a country (a real landmass of its size would implode due to intrinsic pressure), but it is also too far South to stay on our planet. As the Earth is a spherical formation, any continent of its size and location would fall directly off of its surface downwards (forming another satellite, similar to how the Moon was created) over the course of several hundred years. Antarctica would have suffered this fate, too (and, indeed, some parts of it have) had it not had a low organic population, causing its artificial centrified geomagnetism to keep it firmly anchored on land (as a tack to butter). The 'platypus' is also an impossibility, as its beak (containing high concentrations of opaline silica) would poison the animal's bloodstream, causing it to solidify into solid quartz during development. Kangaroos are actually derived from northern Africa, and have their own symbol in Egyptian hieroglyphics. Any pictures you may have seen of the 'continent' are most likely taken from the doctored albums of Sir William T. Ytterby, a Swedish explorer known for his habit of falsifying data. (The element with atomic number 70, Ytterbium, is actually named after him- it was only discovered after the naming of the element in 1864 that his data was completely fabricated.)
I hope that this will clear up any and all misconceptions you, the reader, may have on the existence of Australia. Modern science shows that Australia, however tempting it may be, simply cannot exist.