Last week's Indigenous constitutional convention, which culminated in the Uluru Statement from Heart, called for a Makarrata commission to supervise a process towards a treaty, or treaties, between the nation and its Indigenous people. In New Zealand, the Treaty of Waitangi was signed at the beginning of British settlement, more than 175 years ago. It recognised Maori ownership of their lands, ensured their rights and is considered a founding document of the nation. How did this striking difference come about?

The key is in the different timings during the development of modern international law, which was undergoing major change when Britain claimed half of Australia in 1788. Then, the major source of that law was custom. Treaties were considered a minor part and, even, not law itself but merely agreements between sovereigns.

International law's main authorities were the books of legal scholars. Britain's first such English-language texts were by William Blackstone, who wrote in 1765: "Colonies ... are either such where the lands are claimed by right of occupancy only, by finding them desert and uncultivated, and peopling them from the mother country; or where, when already cultivated, they have either been gained by conquest, or ceded to us by treaties. And both these rights are founded upon the law of nature, or at least upon that of nations."

Three years later, Captain Cook's royal instructions were that, if he found New Holland to be inhabited, he was to "take possession of convenient Situations in the Country in the Name of the King of Britain" and "with the consent of the Natives"; but, "if you find the Country uninhabited, you are to take possession of it for His Majesty ... as First Discoverers and Possessors".

Although noting regularly in his journal the land was "inhabited" and "not without inhabitants", Cook asserted it "was never seen or visited by any European before, and, in a later deleted phrase, "therefore by the same Rule belongs to Great Britain."

No one in London quibbled over the discrepancy between his instructions and reports. So, the king's commission of 1786 appointed Captain Arthur Phillip governor of "our territory called New South Wales".

Notably, Blackstone had used the narrower term "uncultivated" not "uninhabited". He derived this principle from Emer de Vattel, who had been translated into English only seven years before (1758). While some legal authorities, especially from Catholic Spain, acknowledged rights of the Americas' indigenous peoples, Blackstone preferred the Swiss-born Protestant's argument: "The cultivation of the land is ... an obligation, imposed by nature on man" and those who still lived only hunting or herding, used more land than they "would deserve with honest labor" and, thus, could not complain if "more laborious" and overcrowded nations occupied it.

British officials did not consider this question before 1788's settlement but, in 1819, Britain's paramount Crown Law Officers advised that NSW had been "possessed by His Majesty, not having been acquired by conquest or cession, but taken possession of by him as desert or uninhabited".

As for the rest of New Holland, the Admiralty's Sir John Barrow said in 1824 that Britain should not "hesitate a moment" to take it, as "our greatest commercial enemy" in the East, the Dutch, had a "justifiable plea", given their "original discovery" was "beyond dispute". He continued: "Indeed, I believe it is permitted though not provided for (that I am aware of) in the Law of Nations, that occupancy is a stronger title than priority of discovery; but be this as it may, in the present instance our own conduct [claiming NSW] might be quoted against us." In 1829, Western Australia was taken.

By 1840, however, much had changed. Modern treaty-making, now going beyond traditional subjects of war, alliance, peace and dynastic succession, had taken off, with new commercial and, often linked, anti-slavery treaties. Domestic law-making associated with such treaties had started. This began in mid-18th century Continental Europe with Britain slow to join, until so forced by the American Revolution.

Driving this was what the Anglo-French joint declaration of 1814 against slave trafficking termed the "public voice" which, even before enfranchisement, was finding expression through the earliest non-governmental organisations, such as the Anti-Slavery societies, which had succeeded with Britain's anti-slavery acts of 1807 and 1833, and the related Aborigines' Protection Society of 1837.

Britain now led with anti-slave-trafficking treaties. For not only did its governments listen to that "public voice", but the head of the Colonial Office in 1840 was Sir James Stephen, whose father had been the legal adviser behind William Wilberforce's anti-slavery act.

In 1831 when Maori chiefs, influenced by British missionaries, petitioned for British protection, Stephen deliberately drafted the response to express Britain's recognition of New Zealand as an "independent", "foreign" country. And in 1839 when the New Zealand Company, seeking to buy land in what it said was already British territory, Stephen said it was either "ill informed" of or "ill-disposed" to the facts.

There were other pressures. That year, a British subject of French background tried to assert sovereignty over his NZ land claim. France, whose whalers were there, was already organising a small settlement, supported by a warship, and was feared to be planning a settlement in Australia. Britain warned France it held Australia "exclusively" but felt it could not yet oppose a NZ settlement.

The US was rumoured to covet the Sandwich (Hawaiian) Islands and its whalers, supported by a newly-appointed consul, were also asserting their rights to fish NZ waters, as they were off Western Australia. On claiming NZ, Britain advised both colonies not to admit them within three miles, forcibly if authorised.

Although the Admiralty coveted NZ flax and timber, and British commercial interests sought whales and seals, Britain was not seeking every Pacific island, having rejected an earlier, similar plea from Tahiti due to cost.

So, Britain's decision to secure sovereignty over NZ was partly motivated to protect the Maori. Indeed many documents show officials talking about outrages against Maori; there was also a bill, organised by Stephen in the Colonial Office, brought to the House of Commons in 1832 to punish crimes of British subjects in NZ. It was rejected because Britain was not seen to have any jurisdiction over the "foreign" NZ.

Britain was also hoping for new commercial treaties with its great global rivals - France and the US - and so did not want to base its competing legal claim over NZ on a blunt unilateral assertion under debatable customary international law. So, as treaty signings by the chiefs proceeded, Britain pronounced its sovereignty over NZ: by treaty over the North and custom for the South and Stewart islands. .

In London, Stephen commented "how much wiser was the course taken of negotiating for a cession of the sovereignty, than would have been the course of relying on the proceedings of Captain Cook, and the language of Vattel in opposition to our own Statute Book", arising from the treaty.

While differing perceptions of Maori and Aborigines were also an issue - "They did not live simply by the chase," quipped one British official - a more developed international legal regime was the ultimate source of the difference.

In 1788 a vast, potentially wealthy continent was seized on a vague legal basis promoted by a handful of writers who never left Europe and knew nothing of the societies already living there.

Gregory Pemberton is writing an international legal history of Australia.