G-d must have created them, and then put them there, which is both special creation and special delivery.
Because they evolved there and they're not too good at swimming.
You're forgetting this minor detail called Noah's Flood. If this truly was a global flood, the kangaroos would have to swim to the Middle East, get on the boat, get off the boat, and swim back to Australia. Unless you think God miraculously re-delivered them to save them the travel time.
You might point out to God that he forget to put the opossum back with the rest of the marsupials a few thousand years ago.
Isn't your argument self-contradictory? On the one hand, you can't imagine how kangaroos made it to Australia after the flood, and on the other hand you are pointing out that marsupials exist on more than one continent.
And to mimic the AiG site: It may be, it is possible that, the kangaroos were carried on faerie wings to their new home.