Here's a question that I would like to see discussed. If the ark existed and if the entire world was flooded and if all the animals that needed to be out of water were on board, how then did the species get back to where they are today? There are species that are native only to particular islands. How did they get there?
Certainly they didn't walk. Not over water. Assuming the Bible's timeline is correct and the flood occurred only within the last 10,000 years (or even the last 100,000 years) that's not enough time to get the entire planet repopulated to the levels that exist today with the level of complexity and integration that exists today.
The only logical explanation to species dispersment across the world that I've seen, short of God magically making species with no wings fly, is continental drift. The problem with continental drift is that by definition it blows the Noah's ark story to bits due to the length of time it would have taken. To believe in Continental Drift and Noah's Ark would mean placing Noah's Ark back hundreds of millions of years ago.
There is an interesting verse in Genesis 10:25. It says after the flood, In the time of Peleg, the Earth divided. I've heard some speculate this was the division of the continents.
There are several possible explanations. Many such species can live in water. For the small species, they could float on mats of vegatation. When the flood waters go back down they are in about the same place they were. This would work fine for little stuff, but for anything bigger than a monkey I would say it is unlikely.
Australia is not as big a problem as it seems. Placental mammal fossils have been found there, they just, for some reason, died out. It could be that both kinds of mammals were carried on the ark, and both spread from there. For some odd reason, placental mammals did not make it in Australia, and MOST marsupials did not make it eleswhere. Both kinds were in both places before the flood, but AFTER the flood one kind dies out in one area, the other kind dies out in the other.
Microevolution, oddly, actually greatly strengthens the case for Noah's flood. Through loss of information, speciation occurs. That means many species that are unique to islands could have evolved after the flood. This would also reduce the number of animals the Ark would had to have carried. The Komodo dragon then, could have evolved from smaller members of their family after the flood, in response to conditions on those few islands. Of course, immature komodo dragons could have floated on mats of vegatation for a while too. The same for say, giant lemurs in Madagascar. It is possible that livng things can change within limits quickly, but the limits are real. Mircoevolution can happen within 10,000 years, marcoevolution never or almost never.
There is also the possiblity that the "whole world" being flooded was the world of the patriarchs and not the whole globe. I don't lean to this myself, except maybe in the idea that every continent was affected, but not every part of that continent. If asteriod impacts sent up millions of tons of evaporated water and formed Tsunamis that overran much of the globe there might have been some small corners of the globe untouched. Wrangel Island, with the late-living mammoths comes to mind.