Earthquakes, volcanoes erupting, and volcanic hotspots caused the rest. The activity of the tectonic plates at this time split Pangaea into the various continents.
It is true that the Australian animals have distant relatives from the northern continents, but how about the ones at the equator? They aren't much closer. This is because the land bridges(Panama, the Middle East) have let the animals from each group of continents mix with each other. That means the only two continents with very unique animals are Australia and Antarctica. Probably because many species died out in Antarctica in its cooling temperatures, Australia has the strangest animals of all.
These are some accounts from the European explorers and the approximate year of the animal's discovery:
1770: "Europeans have long regarded kangaroos as strange animals. Early explorers described them as creatures that had heads like deer (without antlers), stood upright like men, and hopped like frogs. Combined with the two-headed appearance of a mother kangaroo, this led many back home to dismiss them as travellers' tales for quite some time." A common myth about how the kangaroo got its name is that one of the English explorers asked a native what the strange hopping animals were called. The native replied 'kangaroo', meaning 'I don't understand you'. In the myth, the explorers thought that was the animal's name.
1783: The name 'Kookaburra' comes from the kookaburra's call, a harsh laughing. The English explorers sometimes thought the birds were mocking them.
1798: The platypus is probably the most unique Australian animal of all. When the first sketch was sent to England, British scientists thought it was a hoax. It was later thought that someone had sewn a duck's beak onto a beaver-like animal.
1798-1810: When some of the first reports of the Koala came out, the explorers compared it with a sloth.
There is no doubt that the English explorers thought the Australian animals were strange. This is because they were unique. And there are no similar animals in the world to the Tasmanian Devil, the Spotted-tailed Quoll, and other Australian species we should help and protect.
1. Wikipedia, 'Kangaroo'