Umm... It isn't the "founding Document" of New Zealand. It was never ratified and wasn't even considered important enough to look after until the 1970's. Nor is it legally binding since it was never ratified and never made part of New Zealand law.
You need to learn the history of it yourself.
This has been a major point of legal contention since the 1840s.
One big problem is that Soveriegnty over New Zealand has to have been transferred at some point from Maori to the British Crown in order for modern New Zealand to exist as a legal entity. It is not enough to assume it must have happened at some vague and undefined point. The Sovereignty of New Zealand was recognized to exist in the Maori people when the 1835 Declaration of Independence was recognized by the UK, the USA and others. Therefore if there is not a legal transfer of sovereignty, sovereignty would remain with the United Tribes of New Zealand. Very few people seriously argue this is the case, so the question is, when did the transfer of sovereignty take place?
It is commonly held to be the Treaty of Waitangi that was the mechanism by which sovereignty was transferred from the United Tribes of New Zealand to the British Crown. There is no ther specific event or document whic is claimed as the mechanism forr the transfer of sovereignty. If the Treaty is not binding, then sovereignty was not officially transferred and presumably no law ever passed in New Zealand under Crown authority is legally binding.
There have been various court cases since the 1840s to decide if Treaty principles were binding on the Government of New Zealand. One of the earliest in 1847 decided the Treaty was binding, the judge deciding in R vs Symonds:
'Whatever may be the opinion as to the strength or weakness of Native title ... it cannot be too solemnly asserted that it is entitled to be respected, that it cannot be extinguished (at least in times of peace) otherwise than by the free consent of the Native occupiers. But for their protection and for the sake of humanity, the government is bound to maintain it, and the courts to assert, the Queens exclusive right to extinguish it...The assertion of the Queen's pre-emptive right supposes only a modified dominion as residing in the Natives...This technical seisen against all the world except the natives is the strongest ground whereon the due protection of their qualified dominion can be based.'
However by 1877 New Zealand had it's own court system and when Wi Parata sued Octavius Hadfield, for breach of contract and violation of Treaty principles, Supreme Court Judge James Prendergast held that not only was the Treaty not binding but that it was "a simple nullity". To justify this he added with quite breathtaking cynicism that "In the case of primitive barbarians the Government must be the sole arbiter of it's own justice."
In the case of Te Heuheu Tukino v Aotea District Maori Land Board which went before the courts in 1938 and was heard by the Privy Council in 1941 decided that the Treaty was valid for transferring sovereignty - but not binding on the New Zealand Government unless incorporated into New Zealand statutory law.
Since the mid 1970s, as longstanding Maori issues gained more attention from the general public, acknowledgement has been made in New Zealand legislation that the Treaty is NZ's founding document and that NZ law cannot run counter to Treaty principles.
The 1987 New Zealand Maori Council v Attorney General case established the principle that if the Treaty is mentioned in a piece of legisation it takes precedence over any other part of that legislation in the event that they conflict.
The 1990 New Zealand Maori Council v Attorney General case established that Treaty was (or could be)relevant even to legislation that does not mention it.
If the treaty is not NZ's founding document, then a fair question would be to ask what is?
It's also inaccurate to say that no one considered the treaty important before the 1970s - large numbers of New Zealand citizens, especially Maori, considered the Treaty and it's legal ramifications crucial to the future of the country.
Almost uniquely, New Zealand was colonized by European settlers with the consent of the indigenous people, who had a written guarantee from the Crown that their property, their rights and their culture would be protected
by law. That the attempt is being made to follow through on this promise after much historical hypocracy and injustice should be a source of pride for New Zealanders.