fire wall, how else would you describe an organised large-scale campaign to possess territory that is already occupied without the consent of the existing inhabitantss? There was no treaty, no terms, no negotiation, no recompense, it was just forcefully taken.
What may or may not have happened 60,000 years or so ago doesn't change what the British did after 1788. Two wrongs don't make a right.
Small point, plate tectonics is a rather slow mode of transport. Australia existed as a separate continent well before humans evolved and was last connected to a common landmass with Africa around 150 million years ago.