Ngāti Kahungunu has joined Ngāi Tahu in court action seeking to share control and management of freshwater with the Crown.
The iwi are working together to have Ngāti Kahungunu's rangatiratanga over freshwater recognised, including at the Mohaka River in Hawke's Bay.
Rangatiratanga over water means the local iwi has rights, responsibilities and obligations relating to the freshwater in its takiwā, including doing what it can to stop the degradation of waterways and the environment.
"These issues can be addressed only through direct engagement between the Crown and the iwi whose freshwater are most affected by years of neglect," said Ngāti Kahungunu Iwi Inc chairman Ngahiwi Tomoana.
He said in Treaty land settlements the Crown has maintaned that Ngāti Kahungunu hapu still have rights over freshwater.
"But in trying to determine those rights after the past 10, 12, 15 years, they've dodged the issue, and haven't found any determination or definition around that," Tomoana said.
"We believe that there is a significant bank of legal precedent that will support our claim."
He added that freshwater degradation is as important as any other issue Hawke's Bay faces, including the housing crisis.
"We're taking our kaitiaki status seriously in terms of assisting government to make better decisions based on our tikanga and our custom," Tomoana said, noting that too often economic priorities take precedence over the environment.
"We're spearheading it on behalf of hapu, who do have the mana of the wai, and they do live beside the waters, and they do monitor it every day."
He said the iwi would be in a far better position to make a difference if it worked alongside the Crown on issues, rather than having to react after every drought, pollution event or campylobacter outbreak.
Ngāti Kahungunu's takiwā stretches from the Wairarapa to Wairoa and includes Wairoa Moana (Lake Wairarapa), Lake Ferry and the Ruamahunga river.
Hawke's Bay Regional Council chairman Rex Graham said the council expected Ngāti Kahungunu to join the claim, but it was still a big moment.
"They told us they would, and they have, they've done exactly what they said they were going to do," he said.
Graham said the iwi have some frustrations that change is not happening fast enough, which he completely understood.
"I think a huge part of our community has the same frustrations," he said.
"It's a big challenge because it's taken us 100 years to get to where we are."
Despite sharing those frustrations with the pace personally, Graham said the regional council is putting in every resource to make it happen.
He added that Hawke's Bay are in front of the rest of the country in a way because of partnership with tangata whenua on the Regional Planning Committee.
"We're going as fast as we can. It's not as fast as I would like either, it's just going to take time," Graham said.
"We are very strongly supportive of tangata whenua being heavily involved in issues around water, and all natural resources."
The local iwi had previously backed Ngāi Tahu in their legal proceedings, which began last November, when the South Island iwi lodged a statement of claim in the High Court at Christchurch.
At the time, Tomoana said the state of the nation's river and whenua had been so degraded over the past 50 years an urgent response was needed, because government regimes had been slow to address the issues.
"We want to ensure that Papatuanuku and our rivers and our waiora are nurtured from now on to support successive generations over the next thousand years of Takitimu existence."
The claim from Te Runanga o Ngāi Tahu - the representative body of Ngāi Tahu - also seeks costs, as well as declarations that recognise the Crown's failings, namely breaching its good faith obligations.
In December when Ngāti Kahungunu came out in support of Ngāi Tahu's claim, regional council chief executive James Palmer said the council recognised the enormous importance of freshwater as a taonga for Maori, and its centrality to whakapapa.
"As such the management of freshwater is the principal focus of our work with tangata whenua through our regional planning committee and Māori committee of council.
"By law the council must give effect to 'Te Mana o Te Wai', which demands close collaboration with tangata whenua in developing the management response to protect and restore our freshwater."
Palmer said the council is "ramping up its capability to support this".
The regional council agreed with Ngāti Kahungunu that many of the region's waterways were badly degraded, Palmer said.
"The council is doing everything within its existing resources and powers to address this degradation but achieving our shared goals will take an enormous effort by the whole community for several generations to come.
"The institutional arrangements for managing water are determined by Parliament and ultimately any further changes to these will need to be determined by the Crown and enacted by Parliament."