I suspect most people aren't familiar with the background here, so I'll provide some:

In 1840, the British Crown signed the Treaty of Waitangi with a number of Māori chiefs (Māori being the indigenous people of New Zealand), establishing the land as being under British governorship or sovereignty. The Treaty was necessary because the UK had previously recognised a Declaration of Independence of New Zealand.

There were two versions of the Treaty signed, one in English and one in the Māori language (te reo Māori). It was a hastily assembled document, and the Māori translation did not exactly match the English text. While the English version guaranteed Māori "full exclusive and undisturbed possession of their Lands and Estates Forests Fisheries and other properties", the Māori version of the same clause guaranteed them "unqualified sovereignty over all their lands, their villages, and their taonga (usually translated as 'treasures')". In exchange the British government received "Sovereignty" (in the English version) or "kawanatanga" (in the Māori version, transliterated from "governorship", and understood by analogy with the position of the governor of New South Wales, which at this time included New Zealand).

Following the signing of the Treaty, New Zealand became a colony of the UK. Various disputes regarding the sale of Māori land (e.g. settlers being confused about the distinction between permission to settle on land and ownership of that land; tribes selling the land of another tribe they conquered, or their own land that had been seized by another tribe) eventually led to the New Zealand land wars between Māori and the New Zealand government. Following this, the colonial government passed the New Zealand Settlements Act 1863, confiscating a great deal of land from Māori, whether they had participated in the "rebellion" or not. From this point on, the government abandoned any pretence of abiding by the terms of the Treaty. The document was declared by the courts to be "a simply nullity".

The government continued to shit all over Māori for ages and ages. Then in the mid-'70s, the government passed the Treaty of Waitangi Act 1975, which established the Waitangi Tribunal, which was empowered to investigate possible breaches of the Treaty and suggest possible remedies. A decade later, the State Owned Enterprises Act 1986 included a section that said "Nothing in this Act shall permit the Crown to act in a manner that is inconsistent with the principles of the Treaty of Waitangi". Since then, the power of the Treaty has grown thanks to government legislation (though it still lacks any independent legal force) and it is now a major part of New Zealand's constitutional structure.

Waitangi Day is a holiday created to commemorate the signing of the Treaty.

Today, various New Zealand-based shitlords like to complain about "special treatment" received by Māori. One particular source of ire is Waitangi Tribunal settlements. Iwi (tribes) are able to take the government to the Tribunal for past breaches of the Treaty, and obtain redress - often in the form of the return of confiscated land.

And for extra fun, when the linked commenter says

I would love to know what made them colonize from Asia into Polynesia and then onward to new zealand..... and then eat the natives

He is referring to a old racist myth concerning pre-European New Zealand. Supposedly, prior to the arrival of the Māori from Polynesia, New Zealand was inhabited by the Moriori, who were wiped out and "eaten" by the Māori. This myth was used to justify the confiscation of land, on the basis that "the Māori just stole it from the Moriori anyway, so it's not really theirs in the first place". This does not even begin to resemble the truth, and has been discredited for decades. In reality, the Moriori are the indigenous people of the Chatham Islands, and are descended from Māori following the settlement of New Zealand. In spite of the fact that it is patently false, shitlords love to repeat it ad nauseum whenever they are discussing Māori or the Treaty.