From another guy who doesn't like those mean scientists and their dogma: (Link) (I bolded my favorite part.)

"Prior to the flood, the earth may have been one large landmass. Australia did not exist in its present location. Our present continents are shaped as a result of re-disposition of flood sediments, and receding flood waters.

After the flood there were land bridges (because of a lower sea level) that connected many of the continents.

Years later, the glaciers started to melt and the water level rose. This caused many land bridges to disappear.

The animals that were on these continents would be stuck there. Keep in mind too, that many animals were brought to the United States by explorers. They did not travel there themselves. The same may be true for animals in Australia.

For the animals that did migrate on foot, keep in mind one kangaroo would not have to hop all the way to Australia. A group could travel, dying and reproducing for many years along the way. There was probably a lot of trial and error looking for a climate that suited them, in combination with a amble source of food. It's not as if they had Australia as a goal.