If you are looking for some Maori Art, the arts centre is a very good place to buy something genuine. Prices aren't cheap though, but if money isn't too much of a problem I suggest buying it here. It looks great and the artists at this particular place make everything in front of your eyes.
When you are entering into Maori cultural exhibits, the visitors must be invited in to join the community before entering. Your group must select a chief or king for the group, and then that leader must meet with the leader of the Maori tribe. When presented with the opportunity to enter, the visiting leader must show that he and his people are not a warring party, and must accept the gesture peacefully.
On our trip to Ko Tane, Sarah's father got to be the chief for our visiting tribe. This worked out well for us as a whole, because it meant his entire group got to sit in the best seats, enjoy the first bite of the Hangi, and the chief even got to rub noses with quite a few Maori women!
If you go to Ko Tane, I would recommend volunteering for chief duty!