D Day was an invasion. Japan invaded the Malay peninsular. As did the Normans in 1066 against the poms. This is achieved by one apposing army challenging another with the winner taking control.
The first fleet was not an army be it that they had some soldiers as protection and to keep control of the convicts. They were essentially squatters. They came and helped themselves to the land. Yes there were some uprisings and in some areas there were not. Australia was not settled on Australia day. Only Sydney cove was. The rest of the country was settled over the next 100 years,bit by bit.
There was no Aboriginal nation, but a vast network of individual tribes all across the continent many who never knew many of the others existence.
Unfortunatley for the indigenous population this was always going to happen sooner or later as it did all around the globe. Imagine if it was the Japs that got here first.