Depends what your definition of 'terra nullius' is, and whether you believe that Aboriginal people were 'uncivilised' and if they were uncivilised then can another country claim legal settlement of that land for that reason.
The debate will never cease.
In the thousands of years Aboriginal people had here, it's a pity they didn't unite in greater numbers, and have the foresight to build a few permanent structures as symbols of a united people, both for their own reasons and in anticipation of invaders from across the seas.
Upon seeing obvious signs of a united people, impressions would have been different. But they didn't have a sense of ownership of the land. It never occurred to them that other humans may come and want what they had. They were unprepared.
It puzzles me why they couldn't envision such an invasion, when they would invade each others tribes routinely. They were not strangers to war and conflict including turf wars over hunting grounds. But they failed to unite as one people, probably due to the huge distances in Australia, but still... 40,000 years was plenty of time to get organised.
The defining feature of an immigrant is the act of immigration. Nobody born an Australian is an immigrant, regardless of whether their ancestors arrived by aeroplane, by boat, or by foot and canoe, unless they've chosen to emigrate. There is no technicality by which they can be considered an immigrant.
Lets not forget that all Australians except for Aboriginal people are technically immigrants.