Firstly many aboriginals dont want an Australia day regardless of the date, secondly many dont even want the country to be called Australia so doesnt matter what you do they are going to protest.
Thirdly you need to look up the legal defintion of the term terra nullius 'land over which no previous sovereignty has been exercised.'
Finally Australia wasnt called Terra Nullius until there was land dispute in NSW around 1820 if my memory serves me well where a settler was trying to purchase land from an aboriginal...ie 4 cows for 1000 acres type of thing so the govt needed to step in.
