Q. My son asked me how the animals which appear to be indigenous to Australia only (for example), arrived there after the flood? Were they there before the flood? Were they destroyed by the flood? Were there kangaroos on the tayva?

A. Whether all animals worldwide died during the Mabul may be open to discussion. Fish were not included in the decree of destruction by the Mabul as Rashi (7: 21) quotes from the Talmud, (Sanhedrin 108a), See question 1469 if Noach actually saved the fish.
Horav Shlomo Miller Shlit’a suggested that maybe animals in lands uninhabited by humans did not perish.
Another possibility is that most lands including Australia, were then joined as one single land mass. Maharitz Chiyois (Nidah 23a) teaches that the Mabul caused great geologic disturbances that created mountains and valleys where previously there were none. Bereishis Rabbah (28:3), Ramban (8:11) and others mention that the very foundations of the planet were affected during the flood.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a