I'm aware that the Treaty of Waitangi was a major factor but I'm interested in why and how the Maoris managed to negotiate a relatively fair treaty (on paper) and receive things like guaranteed seats in parliament whereas the Aborigines couldn't.
Wow, this is a multivaliant question. As in - you are literally asking about everything that is New Zealand culture! And everything that is not New Zealand culture as well, which is a pickle to answer. Great job! (No sarcasm - asking questions which raise more questions than answers is hard.)
But first we've got to do the traditional historian thing and stop and ask ourselves what those other questions are, okay? No dramas - just stick with me. Or skip if you get board, no worries. But what does it mean to be "treated better"? During what time period? Why have we framed the question from a position of power? (Ie - why not ask why the NZ Pakeha were treated far better, or ask why the two sides were able to come together to form a partnership. Asking it this way assumes the European settlers were always, and still are, in a position of power over how people are treated in NZ, which is true and not entirely true at the same time.)
Since you've asked from the perspective of the British (and French), we'll answer it from that perspective, but next time stop and ask yourself how you're framing your worldview when you ask questions, okay?
From the British perspective, there is actually a broader answer for why the Imperial government believed the Maori better than the Australian Aboriginals (FYI Aborigines is a mild racial slur in Australia - best not to use it). New Zealand was colonised starting roughly 1830, which is after the enlightenment took proper hold. Australia began to be settled roughly fifty years earlier, in about 1780.
There were all sorts of new things going on during the enlightenment - one of them was the categorization and hierarchical-isation of race. This was in line with, and an extension of, the classical concept of a "Great Chain of Being," where every thing that existed in the universe had a particular place in relation to all the others. Think of it like a pyramid scheme. The "British" racial construct was put at the top of this Great Chain, of course, but other races were pretty good as well, and then all humans were better than all animals, which were better than all plants, etc.
The Maori were placed into a relatively privileged place in this "Great Chain." They were seen to have 'civilised' things like a clear tribal structure (chiefs), property ownership (the British just assumed it was their own kind, which will cause problems later), duty and honour (weirdly, the British seem to be mostly right about this), settled homes, a single language (albeit with multiple dialects), and large-scale organised warfare. (The AA seem to lack things like this, being a nomadic and small-group culture. A lot of the things that they did have and do were not seen by visitors.)
Anyway, to cut a long story short, the Maori were a 'noble savage'. They were at the point where signing treaties was not only possible, it was desirable, because Maori would be uplifted by contact etc etc blugh blugh you get the picture. It's still racist as hell, but its the kind of racism which means you get protections under the law. Indeed, with the signing of the Treaty the Maori became full British citizens, entitled to all the same protections, rights, and responsibilities - they even got the franchise with all other British men when parliament was established.
The Aboriginal Australians are not one people. They are highly fragmented in language, culture, and geography. They tend towards the nomadic, their warfare was small-group, and things like property ownership can be hard to discern if you're a British person. They were not classed as a noble savage, they were classed basically as only one step above the animals on the chain. And not a short step either. Quoting user /u/TasfromTAS
[while you couldn't get an AA hunting licence - a common myth in AU]...that's not to say settlers at various times weren't able to kill indigenous people with legal impunity, they certainly were. At times martial law would be declared and roving parties of settlers were authorised to kill any indigenous person who would not comply with demands to move to designated areas.
Legally speaking, in contrast to the New Zealand situation, the consitution of Australia literally has racism built into it. Section 51(xxvi) of the Australian Constitution allowed the government to make laws based on race. Hell, if you want a clearer contrast between how the AA were seen and how the Maori were seen, the Australian Commonwealth Franchising Act of 1902 explicitly recognises the Maori right to vote in Australia (mostly because they wanted NZ to federate with them eventually and were worried it would be a sticking point - but still. Here is the transcript from it's second reading
I hope I gave you some shape to your discussion, although like I said the question is HUUUUUUUGE.
TL:DR; cultural catagories back in the day.
Hi, you may be interested in this post
There is a previous post from a year ago that covers half of this question. Although it seems to result in more questions than it answers. Possibly /u/CrossyNZ can help with the other half of the equation?
https://www.reddit.com/r/AskHistorians/comments/2x7vgy/why_were_indigenous_australians_generally_treated/