Moa were, for all practical purposes, not domesticable. All species of moa (including the small ones) took about 10 years to reach breeding age. That didn't matter too much in an environment where they had few natural predators, but it left them extremely vulnerable to human hunting. It also meant that no-one was really going to succeed in proper domestication - 10 years is rather too long a waiting period for the next generation of captive moa.
Not exactly. Moas could provide eggs, apart of meat, something that could justify the option of keeping domestic herds. And there is also the possibility to use them in locomotion or as burden beasts.
However I agree that this envision was too much for the first Maori, and simple hunting was the easiest option, considering that they were not challenged by the competence of other human populations. Domestication of animals have been usually triggered by high competence between human groups for the same animal resource, which made some groups to try domestication, thus ensuring a secure resource unaffected by competition on hunting fields. And these conditions did not happen at NZ as well as did not happen in all Oceania. The expansion of the Polynesian people through Oceania caused the extinction of 10% of the species of birds (normally by hunting), a number far superior than the one caused by post-industrialization humans worldwide up to this date.