Woah - nice and complicated!
Yes, I think that you will need to be in NZ for those two years (though you can of course leave for hols, as long as you have the six months of each year in NZ), and then you can come and go as you want. If you can't spend that time for any reason (i.e. if your wife's mother became ill, and she had to go back to care for her), then she could still get an extension to the visa (off the top of my head because I've shut down the browser link!) for 2 weeks, 6 months or a year, depending on the circumstances. But if she can wait for the 2 years, then it would be better, because that then would get rid of all complications