But a spokesman for the King, Tuku Morgan, said invitations for Mr Key and his MPs were never considered.
"This is a national summit on water for our people to ... have a constructive discussion in our time and in our own space."
The meeting would allow Maori Council co-chairman Sir Eddie Durie to spell out the case to the tribunal "because many of our people have not heard it other than what they've read in the paper".
But several Maori politicians will attend, including from the Mana Party and Labour's Hauraki Waikato MP, Nanaia Mahuta, the King's cousin.
The Herald understands Green Party MPs will attend if invited.
Mr Morgan said organisers had yet to have talks with the Maori Party. But co-leaders Pita Sharples and Tariana Turia were last night noncommittal at best.
The party's attendance was "still to be determined", Dr Sharples told 3News. "We believe this is a thing that iwi and hapu are going to have to work out themselves."
Mrs Turia said: "At this point I don't see the point of us going."
Meanwhile, Mr Key said it was "extremely important that people don't get confused" about the talks his Government was to have with affected iwi about the Mighty River sale over a five-week period. The purpose of that consultation was "to confirm the point that the sale of shares in those companies in no way affects the rights and interests of Maori in water".
Maori Council deputy chairwoman Rahui Katene said Mr Key's comment called into question the Government's good faith. "If he's going into any legal action saying he showed good faith by having consultation it's going to be very easy to show that no, in fact this was not consultation."
Maori Council lawyers were primed to file a legal challenge to the Government's Mighty River sale plan, she said, but were unlikely to do so until after a September 15 meeting.