Two days ago, Australia celebrated its national day. Ferryboats raced on Sydney Harbour. Families gathered for barbecues. The youth broadcaster, Triple J, counted down its 100 most popular songs of the past year. Across the country, citizenship ceremonies were held. The prime minister made his annual address to the nation. More names were added to the official honours list.

On Australia Day I thought of various things and of one man in particular: Eddie Mabo. A native of Murray Island in the Torres Strait, he left school young and worked as a pearler, cane cutter, railway fettler and gardener. He had seven children with his wife, Bonita, and they adopted another three. Mabo held an unschooled but unshakeable belief: that he owned his land on Murray Island by traditional right, in the face of the universally accepted legal position that the land had been acquired by the Crown long before he was born. This view was the foundation of a classically Australian story, of right defeating might.

Mabo’s legacy is arguably the most important judgement the High Court of Australia has ever written. He fought the good fight but died, sadly, five months before the verdict in the case that bears his name. He changed the course of Australian history, and he did it for no personal gain. He fought simply for a principle he kept believing must be.

Mabo’s extraordinary achievement was to debunk 204 years of flawed legal presumption. Six to one, the judges of the High Court agreed with his fundamental proposition that terra nullius, the doctrine that deemed the Australian landmass to have been legally unoccupied before the First Fleet’s arrival in 1788, was quite simply wrong.

In fact, when Captain Cook sailed the east coast in 1770 and claimed all of it for the British Crown, the land was already owned. Terra nullius was developed as a legal fiction of convenience, for obvious practical reasons but with no basis in law or fact. Thanks to Eddie Mabo, it’s now a historical curiosity.

Which brings us back to Australia Day. Once a year, Australia celebrates its nationhood. The essence of that celebration is what it means to be Australian: what draws into a unified whole all of the disparate strings of the country’s formation and population.

Australia Day is when we pause to acknowledge and retell our story, requiring some consensus on what that story actually comprises. The current reconciliation narrative of broadest acceptance has three parts: the First Australians, British colonisation, and successive multicultural waves of immigration. Largely, we’re comfortable with that.

In the telling, the ever-expanding Australian story traces the formative actions of men and women: sailors, soldiers, convicts, explorers, pioneers, farmers, miners, rebels, leaders and entrepreneurs. On Australia Day we also single out some who exemplify the values of Australian identity and mark them for honours. This is a good thing. It should be celebrated. The problem with Australia Day isn’t the idea but the date.

January 26 marks the commencement of British colonial possession of Australia. That’s how we tend to think about it, forgetting the unavoidable consequence of Britain’s claim to the continent: that the previous owners of the same land were in that act dispossessed. To First Australians, obviously, it was an invasion. We don’t have to stretch our empathy that far to understand the basic point that what happened was a taking, not a giving or sharing, of land.

Apart from the social rationale for the change, I also think there’s an important issue of law involved. Australia Day as currently celebrated is an anachronistic relic of the discredited terra nullius doctrine. In terms of the need to give proper legal recognition to the fact of unlawful dispossession that started in 1788, it’s important we remove the symbolic suggestion that that process was lawful.

Whatever you think about the process of colonisation that followed – certainly there was no chance of Australia remaining in exclusively Indigenous hands, and you may feel that what happened was ultimately for the best – for the Aboriginal and Torres Strait Islander peoples the process was catastrophic.

Devastation of the Indigenous population, and the basic assumption of racial inequality that has underpinned the refusal to allow that Indigenous Australians could ever be treated other than as a “special” case, are inalienably part of our history. At the root of all that is the founding injustice of dispossession, which was racist in its justification and openly understood at the time to be so.

Dispossession denied the truth of prior ownership; consequently, it stripped the owners of their claim to equal humanity. The many horrors that followed would have been infinitely harder to perpetrate and accept had terra nullius never been pronounced. As the High Court put it in the Mabo case, the legal endorsement of terra nullius “provided the environment in which the Aboriginal people of the continent came to be treated as a different and lower form of life whose very existence could be ignored for the purpose of determining the legal right to occupy and use their traditional homelands”.

It didn’t have to be that way, and history has examples of more legally sound approaches such as New Zealand’s Treaty of Waitangi. But the point isn’t to get into a debate about whether Australia’s Indigenous peoples were, from a white-skinned perspective, properly able to assert title to their land. The point is that that land was where they lived. It was taken from them, without conversation or compensation.

Dispossession, historically inevitable or not, was legally wrong. Since Mabo, that legal wrong has been, in principle if not so much in practical reality, made right. Mabo corrects, imperfectly but as best the law can, the mistake that began on January 26, 1788.

There is no reason why we should not recognise that date. It was a signal moment in the foundation of Australia’s ultimate federation as a nation, with parliamentary democracy and the rule of law, and, despite a long battle to keep the place entirely white, set off the waves of migration that have resulted in our becoming the world’s most successful experiment in cultural integration.

But January 26 marks the start of not one but two central themes in Australia’s story: our destiny as a modern, industrialised Western democracy, but also, for those who were here before that day, our history and present continuation of dispossession, discrimination and decline.

It is a sorry fact that the day on which we celebrate our story, and our national heroes, is one that in part commemorates the exclusion of some of us, including other great heroes, from that story.

If January 26 marks the beginning of dispossession and alienation, other dates enshrine steps towards unity. May 27, 1967, represents the moment Australia voted to include Indigenous people in the census – a date on which we moved forward as one people. June 3, 1992, represents the day the Mabo decision killed the fiction of terra nullius. October 9, 1942, saw the Statute of Westminster Adoption Act become law, giving our parliament the freedom to lead without foreign political ties and controls. January 31, 1973, saw the final vestiges of the White Australia Policy essentially dismantled, as the Whitlam Labor government amended the Holt Liberal government’s Migration Act 1966 to remove racial considerations from immigration law.

These events were no less real, and in the full perspective of history will hold no less significance, than the day when we temporarily joined the British Empire. Each of these dates tells part of our story; none represent the whole thing. Another argument, for instance, suggests we take a day at random, free from symbolism and the weights of history – the first Friday in February, for argument’s sake.

We can be bigger than this. We can recognise that it is unkind to insist that the one day of the year when everyone declares their identity as Australians must take place on a date that brings sadness to a significant part of our community. Our history overflows with causes for pride, but it is darkened by the treatment of Indigenous people. I think we can find the heart to give up what is, after all, just a date, and settle on one that instead marks nothing other than how good it is to be Australian.

On Thursday, my law firm of 38 people remained open for business. Our staff voted overwhelmingly to join the campaign to change the date of Australia Day. So have scores of other businesses: non-profits, arts organisations, architectural practices, design companies, investment firms and this newspaper. As a statement of principle, we elected not to recognise a public holiday that politicians refuse to recognise as divisive. In the loudest voice we possess, we sought to make clear that the work Eddie Mabo began is not yet complete. The principle of terra nullius is dead and not mourned: we should stop celebrating its birthday.