I'm not intimately familiar with any relevant statistics, but I'd certainly imagine there are problems with Aboriginal attainment levels. However, as far as I'm aware, a lot of the problems in the US stem from local funding models, and would affect a much larger percentage of the population. And scholarships for Aboriginal students are one of the most notable of the small number of exceptions to the pure use of ATAR. Also, universities often have bonus point systems, so e.g. if you're wanting to get into a course with a 90.00 cut-off but got 86.00, you might benefit from a +5.00 for being living in a disadvantaged area.
I'm not sure that I agree that ATAR isn't a good way of predicting the likelihood of getting a first-class degree/honours - if someone receives a lower ATAR, then sure, perhaps if they had been given a better opportunity during high school they may have fared better, but that's a hypothetical. Simply put, their lower ATAR indicates that they have not performed as well academically than someone with a higher ATAR. Now, we can say that they should be compensated for their lack of high school opportunity by being given a university place despite their lower marks (and to be clear, I support such affirmative action), but that doesn't magically make them as well-equipped academically to deal with the work. Alternatively, we could say that university is about more to society than producing honours candidates. But I'm sceptical of an argument which suggests that somehow present academic achievement isn't a good predictor of future academic achievement.