Key has been at loggerheads with a Maori tribe, the Tuhoe, over negotiations to settle their grievances over land confiscations by European settlers in the 19th century.
During a speech to a tourism conference Thursday, Key joked about having dinner with the neighbouring Ngati Porou tribe, or iwi.
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"The good news is that I was having dinner with Ngati Porou as opposed to their neighbouring iwi which is Tuhoe, in which case I would have been dinner, which wouldn't have been quite so attractive," Key said.
A settlement negotiator with the Tuhoe tribe, Tamati Kruger, told Radio New Zealand the joke was in poor taste.
"I'm just astounded that the Prime Minister can make light of what we regard as a very, very serious situation (over the negotiations)," Kruger said.
"I don't think it's becoming at all of a prime minister."
An MP for the Maori Party, which supports Key's government and has two leaders serving as government ministers, said the joke was unfortunate.
"Well the first thing to say is, it's probably correct, and the second thing is (it's) probably not wise in the current climate," said Te Ururoa Flavell.
Cannibalism remains a sensitive subject in New Zealand, where Maori warriors sometimes ate their defeated enemies until the practice died out in the mid-19th century, according to historians.
