The theft of any cultural artifacts of any country is never a good thing. Any Maori mummy heads that are out of New Zealand should be returned to whatever Maori tribe was robbed. France recently returned 20 Maori heads in 2012. That's a good start. If there are any more out there, they should be returned for a proper burial.
Indigenous people have long experienced the injustice of having their artifacts and sacred objects taken by people outside their culture. Taking sacred objects, particularly human remains, is disrespectful to other cultures. Imagine if someone snuck into the graveyard where your family was buried and made off with some parts! Let native people determine the fate of their relics.