Pixate,
You question is different than SecularFrummy’s. SecularFrummy’s is a question of time, your question is one of ability.
I think there are two possible derachim to answer that question. The first possibility is something along the lines of Rabbi Dovid Brown’s (Z”L) teretz in Mysteries of Creation, that at the time of the mabul the continents were still together and they were only separated in the Dor Haphlaga.
The second possibility is that the mabul was only in the areas of human civilization and did not effect the Americas or Australia at all.