While I think it's right for our generation to look back and say "It's wrong to disposess the lands of aboriginals" I think it's equally wrong to say they were unethical for doing so merely by the fact that they did. I think you have to look at the education someone was given/had access to before deciding whether they made the best ethical decision they could at the time.My point is that we should be able to generalise ethical principles. Actions should not be regarded as ethical or unethical because of the dominant cultural views of a particular period. Rather, there should be abstract principles that act as philosophical guidelines for the judgement of ethics. These principles can then be used to judge actions historically and cross-culturally.
For instance, I regularly eat meat... I do so because I don't think animals have the same cognative faculty to understand pain/their death quite as much as I do (though even then I still find it hard to justify), now if in the future we find a way to unlock the secrets of animal consciousness and it turns out what we've been doing is akin to a few millenia of holocaust-like activity... should I be judged by them as having been unethical? I think not.
I tend to believe the knowledge that aboriginals were equal human beigns on all accounts simply wasn't there.