Scientists nowadays say that all the continents were once one, thus explaining why kangaroos can only be found in Australia. They call this super continent Pangaea. Some Christian scientists consider this idea might be true, a verse in the bible seems to compliment this theory,
Some of them decipher this part of the verse as proof that there was a super continent.
Other Christian scientists believe that the super continent existed after the flood, and yet others believe that there was no Pangaea but that kangaroos got to Australia because of continental drift.
Another theory is that land bridges once connected all the continents together. After the flood, an ice age occurred, and the bridges were exposed and people and animals migrated over them. After the ice age ended, the water level rose, covering the bridges.