The landmark 4-3 majority decision, with seven separate judgments, places Indigenous Australians (meeting the tripartite Mabo test of biological descent, self- identification and Indigenous community recognition as Aboriginal) beyond the reach of the Commonwealth's "aliens and naturalisation" head of power. Rattling parliamentary sovereignty on immigration issues, the ruling curtails the scope of the power relied on by the Commonwealth to remove or deport people from Australia.

The historic decision, with enormous implications for Australian nationhood, centres on two Indigenous Australians facing deportation to Papua New Guinea and New Zealand under the Migration Act 1958.

Daniel Love and Brendan Thoms identify as Aboriginal, yet both men were born overseas; Love is a PNG citizen and Thoms is an NZ citizen. Love has lived in Australia since the age of five and Thoms since 1994. Each has one parent who is an Indigenous Australian, and both held Australian visas until they were cancelled under the Migration Act.

The court determined that Indigenous Australians can be non-citizens and non-aliens - constituting a special, sui generis (unique) status under the Australian Constitution. In their separate reasons, Justices Virginia Bell, Geoffrey Nettle, Michelle Gordon and James Edelman held that it is not open for the Parliament to treat an Aboriginal Australian as an "alien" because the constitutional term could not extend to them due to their special cultural, historical and spiritual connection to Australian territory.

The court wrestled with the deeply unsettling proposition that there may be circumstances in which Indigenous Australians can be deported from Australia. As Justice Stephen Gageler wrote in his judgment, the arguments of the plaintiffs were morally and emotionally engaging, despite him finding against them.

The Commonwealth argued, and Chief Justice Susan Kiefel and Justices Stephen Gageler and Patrick Keane agreed, that it was within the Parliament's power to define who is and isn't an "alien" through Australian citizenship law - that is, Indigenous Australians can be non-citizens or "aliens". Indeed, the Commonwealth stated in its written argument: "It would fracture the understanding of the aliens' power upon which Parliament has long relied, by creating a class of persons of uncertain size and definition who would stand outside both the Australian Citizenship Act 2007 (Cth) and the Migration Act 1958 (Cth)."

Exactly why the government was able to attempt to argue that two Indigenous Australians are not Australian citizens is because there is no citizenship provision in the Australian constitution, and no clear statement about who is a member of the Australian community. It is why the government has been similarly emboldened to amend the Australian Citizenship Act to strip dual citizens suspected of overseas terrorist activity of their Australian citizenship, as there is nothing in the constitution that directly stops it from doing so.

John Quick, a leading constitutional authority of his day, asked during the Australasian Federal Convention Debates, from which the Australian Constitution arose: "Are we to have a Commonwealth citizenship? If we are, why is it not to be implanted in the constitution? Why is it to be merely a legal inference?" His proposals were rejected. As a result, it was left to the "aliens and naturalisation" power, and the "immigration and emigration power", to enable the federal government to make laws over immigration and citizenship.

Until yesterday, the High Court had found that a non-citizen, as defined by the Australian Citizenship Act, is an alien. If the Commonwealth had been successful in arguing that, constitutionally speaking, an Indigenous person can become subject to the removal or the deportation powers of the Migration Act because the person is a non-citizen, it would have reinforced the observation that the Parliament has far too much power to determine the boundaries of membership of Australia.

Such a decision would have affirmed the view that the powers under the Australian Citizenship Act had returned to the pre-citizenship status of Australians being subjects - "subject to" the benevolence and power of the King or executive. Subject status, as opposed to citizenship in a western liberal democratic nation, reflects an imbalance of power of the executive over its citizenry.

READ MORE:

But the majority of the court - Justices Virginia Bell, Geoffrey Nettle, Michelle Gordon and James Edelman - recognised a limit on the Parliament's power to determine the boundaries of membership.

As Justice Edelman wrote: "'Aliens' is not a peg on which the Commonwealth Parliament may hang any legislation concerning citizens according to its own definition." In his view, the sense of identity that ties Aboriginal people to Australia is an underlying fundamental truth that cannot be altered or deemed not to exist by legislation. Justice Gordon started her judgment by recognising the "deeper" truth that the Indigenous peoples of Australia are the first peoples of this country, and that the connection between the Indigenous peoples of Australia and the land and waters that now make up the territory of Australia was not severed or extinguished by European "settlement".

Whether this judgment is honoured by the Parliament and whether the executive seeks to go back to the drawing board to see if there are other heads of power they can rely on to remove non-alien non-citizens, as Justice Stephen Gageler posited as a practical outcome of the plaintiffs' case, will reflect on the moral compass of the Parliament as it confronts the deeper truth that the majority judgment affirms.