"It's a free world. They're more than welcome not to go if they don't want to," he told TV3's Firstline.
"This is the Government's attempt to consult on this very narrow aspect of 'shares plus'. Our view is that there's not a lot of merit to that ... but I guess it's just an opportunity to check that we haven't missed anything."
Asked if it was difficult to consult if there was nobody to talk to, Mr Key said the Government had a legal obligation to consult in good faith.
"And if they choose not to, then well, they can do that on their own advice. We have our own advice on this."
Mr Key said the shares plus concept was "largely pulled out of thin air" by the Waitangi Tribunal.
"It wasn't fleshed out in any major way. We think it's just fundamentally flawed in a lot of areas," he said.
"We're just simply saying look, maybe in that one in a million there's something we've missed, let's have a chat about it."
Mr Key said he was not open to a national solution on water rights, but there was a framework for dealing with individual historical water claims.
Waikato Tainui said it was not attending the hui because it wanted clarity over what other hapu and iwi would do before it embarked on its own talks, RNZ reported.
Ngati Koroki Kahukura said it was not going because it had already told the Government what it wanted.
Tainui leader Tukoroirangi Morgan said no Tainui iwi were going, according to Maori Council co-chairman Maanu Paul.
Finance Minister Bill English, who will attend the six hui in 10 days, yesterday said he was confident the partial sale of assets could go ahead next year if market conditions allowed.
"The Government is confident that in law no one owns water, and is also confident that proceeding with the share offerings will not compromise its ability to recognise those rights and interests that Maori do have in water."
The hui:
- Tuesday 18 September, Tainui waka iwi including Waikato, Hauraki, Ngati Raukawa and Ngati Koroki Kahukura, in Hamilton
- 19 September, Ngati Tuwharetoa and Te Arawa, in Taupo
- 19 September, Whanganui Iwi, in Whanganui
- 26 September, Tainui waka iwi Ngati Maniapoto, in Te Kuiti
- 27 September Waikaremoana Iwi including Ngai Tuhoe, Ngati Ruapani and Ngati Kahungunu ki te Wairoa, in Tuai
- 27 September, Ngai Tahu, in Christchurch