Mr Key pointed to the $40 billion Maori economy, such as Ngati Apa's farming interests, and said they would miss out if New Zealand was not in the agreement.
"You have huge interests now through the treaty settlements process and they're going to get bigger. But nobody owes this country a living. We have to make our way in the world. And Maori have been some of the most successful hungry traders this country has ever produced.
"There is nothing to fear in that."
Mr Key had earlier been challenged about the Trans-Pacific Partnership by Ratana speakers, most notably Rahui Papa from Waikato iwi Tainui.
Mr Papa called for the signing of the TPP - due to happen on February 4 - to be delayed until the public could be better consulted.
"Closed door negotiations don't work for Maori, Prime Minister. One of the snags has been the lack of information and dialogue around this. I ask, no, I implore you, to take the pulse of the people and delay the signing until a comprehensive discussion and debate with all of those across the country can be held.
"We are no stranger to free trade agreements from the 1800s. We want to see the opportunities not only for Maori, but for us as New Zealanders."
Another Ratana speaker, Che Wilson, called for cross-party alignment over the TPP.
"The best way for us to find alignment is to talk to each other. I hope we can stop playing games."
Breaking with tradition, the politicians were welcomed as one large group on to the marae. In the past the different political parties have arrived and gone on to the marae at separate times.
MPs from National, the Green Party, New Zealand First, the Maori Party, United Future all walked together. There was also a very large contingent from the Labour Party - including leader Andrew Little - as this year marks 80 years of the agreement signed by TW Ratana, and then-Labour leader Michael Joseph Savage.
Mr Little used his speech on the marae to criticise the TPP, saying Labour would "stand together with concerned citizens" on the issue.
He also promised to "reinvigorate" the relationship between the party and the Ratana movement and turn it into a relationship that was not simply symbolic. Mr Little announced that Labour planned to extend its existing policy to pay employers the equivalent of the dole if they took on young apprentices so more were eligible.