While most kangaroos can be found in various parts of Australia, it should be remembered that there are some that are found in New Guinea too which are near Australia. According to some reports, kangaroos are only found in the area of Australia and nearby New Guinea because this is how the animals from that point on evolved. It should be remembered that Australia broke free from Africa before.
Those that are in the side of Australia became marsupial mammals which means that they carry pouches. The other animals found in other continents of the world became placental mammals which means that they carry their young inside before giving birth.