> That's even worse though, isn't it? A racist policy favoring indigenous by blood could at least be justified by saying it's correcting for past racism in the opposite direction - however weak that justification is. But what you're describing sounds like granting privileged status to a club, membership in which is dictated by whether you identify with it, and existing members identify with you. Similar to privileging a religious group, but sans religion itself.
I'm not necessarily defending these policies–just attempting to describe them accurately–if one is going to criticise something, it is important to accurately describe the thing to be criticised. This is an issue on which I have mixed feelings and can see both sides of the argument.
But to clarify, at least officially speaking, it is based on blood–it is not enough to simplify identify as indigenous and be accepted by a recognised indigenous community as indigenous, you also have to actually be of indigenous descent – a person with zero indigenous descent is officially speaking not indigenous, even if they identify as such, and even if a recognised indigenous community accepts them as such. The difference from American "blood quantum" laws, is there is no requirement for any minimum degree of indigenous descent – officially it doesn't matter if a person's ancestry is 100% indigenous or 1-in-256 indigenous – but officially a person with zero known indigenous ancestry can't be indigenous. In practice, people with extremely remote indigenous ancestry are unlikely to be able to find a recognised indigenous community willing to endorse their indigenous identification, but standards vary from community to community, and also change over time – sometimes it becomes a very political issue, with certain factions within a community advocating for much tougher membership standards than others, and so it can depend on the outcome of the political battle between those different factions.
In practice, Australia doesn't have any central list of "who is indigenous". Every agency makes that decision for itself. For statistical purposes, the government just uses self-identification, and doesn't require any evidence of descent or acceptance by a recognised community. When it comes to "affirmative action" (reserved jobs, admission to educational institutions, scholarships, etc) and voting rights in representative bodies (such as the "land councils" through which indigenous communities own land), agencies usually require more than just self-identification: generally a letter or certificate issued by a recognised indigenous community body (often a land council), although sometimes other forms of evidence can be accepted instead.
> No way this could possibly go wrong.
There is a lot of controversy in the Australian indigenous community over so-called "box-tickers", who identify as indigenous, despite having only very remote indigenous ancestry, or even no such ancestry at all. The requirement for a letter/certificate issued by recognised community can be a big hurdle for such people, although given the political controversies over the standards for issuing such letters/certificates (and even occasional allegations of corruption), we can't rule out the possibility that some people with zero actual indigenous descent have successfully obtained such letters/certificates. Officially speaking, such people aren't actually indigenous, but the system will treat them as if they were. However, there was a case (in NSW) a few years ago where a woman claimed to be of indigenous descent, and had her claim endorsed by her local land council, which then accepted her as one of its members – a competing faction on the land council insisted her indigenous ancestry was bogus, gathered evidence of her genealogy, filed a formal complaint with the state government agency which manages land councils, and successfully convinced that agency to revoke her land council membership.