The country's third biggest iwi, Ngati Kahungunu, says Ngai Tahu has given its word that it won't follow through with its plan to invest in a Hawke's Bay dam if the tribe isn't happy.
Ngai Tahu has been named as a potential investor in the Ruataniwha Water Storage project.
Ngati Kahungunu runanga chair Ngahiwi Tomoana says Ngai Tahu told the tribe of its plan early on.
He says the tribe's representatives, including tribal chair Mark Solomon, have been in close contact with Ngati Kahungunu.
Mr Tomoana said Sir Mark came to him with his legal adviser to tell his tribe that if Ngati Kahungunu is totally opposed to the scheme, then the South Island tribe will not proceed.
He said he trusts Sir Mark's word and says he knows he'll honour it.
Once the tribe receives all the information - both positive and negative - about the Ruataniwha Water Storage plan, Mr Tomoana said Ngati Kahungunu may in fact support the scheme.