You are attempting to mix evolutionary millions of years with Scripture, and it won't add up. Believe one or the other.

Before the flood, the face of the earth was much different. The land mass was one continent. There was water above the earth and beneath the crust of the earth. When the flood began, all the fountains of the great deep were broken up, and the windows of heaven were opened. The fountains of the deep are the fault lines that we see at the borders of the tectonic plates.

The continents as we now see them are a result of the land sinking under the weight of the flood waters, forming the ocean beds.

THe kangaroos and koalas, most likely did not come from that land to the ark, but they did go from the ark to that land.

After the flood water subsided, much of the water was still frozen at the poles, so the oceans were not as full as they are now. There would have been land bridges going to Australia. As the polar ice caps began to melt, it would have covered those low lying land bridges (the waters are very shallow there) and the land bridges would have been covered with water, making it a separate continent only after the animals had migrated there.