Sorry to bring up such an old topic, but I've been reading this thread and no one has mentioned this possibility...
Has anyone considered that at the time of the Great Flood there may have been only one large continent? In Genesis 10 there follows a genealogy of the decendants of Noah. In verse 25 it says:
"And unto Eber were born two sons: the name of one was Peleg (which means
Division); for in his days was the earth divided; and his brother's name was Joktan.
I would imagine that after such a massive global deluge which involved many geologic factors, there would still be heavy tectonic activity for a long time. It is my belief that the current rate of tectonic movement is nowhere near what it was at the time of the flood. So if the landmasses were not broken apart yet it would be possible for koalas, and any other animal, to make their way to the Ark.
Again, sorry I "dug up a dinosaur"