The deal surrounds a scheme to discharge Foxton's wastewater on to Matakarapa Island - a site of cultural significance in the Manawatū River.
It includes a series of conditions set by affected hapū and a proviso by the council that the iwi signatories would in return drop Environment Court action against the plan.
The conditions negotiated between the council and the iwi include funding to establish an accessway to Matakarapa, land compensation, beautification works, an opportunity to tell the iwi's stories, cultural monitoring and co-management in decision making.
Information provided to NZME by council chief executive David Clapperton specified a breakdown of costs that would apply to the five-year period covered by the deal.
Up to $100,000 has been allocated for fencing on the island, establishing an accessway and contributing towards a whare or information centre, along with $80,000 for projects to tell the history and stories of Matakarapa, as well as for kaumātua visits to the island.
Funding of up to $3000 will go towards establishing a Cultural Health Index protocol and up to $2500 per year, or $12,500 over five years, for monitoring and reporting under that protocol, Clapperton said.
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Up to $5000 will pay engagement costs during construction works, while $2500 has been allocated for potential accidental archaeological discoveries.
The council will also fund up to $20,000 over five years for operational meetings, hui and site visits and $60,000 per year for the iwi to to go towards an environmental manager for iwi.
$25,000 will go towards "development costs such as administrative support, governance meetings and travel costs for both this and other council projects, plans and consultations," Clapperton said.
External funding is also expected to be needed for some aspects of the scheme.
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Currently Matakarapa is inaccessible to iwi as it is surrounded by the river on one side and privately owned dairy farms on the other.
The council had until now refused to disclose details of the deal, including the financial cost of meeting its conditions, however an Official Information Act request by NZME was later examined by the Ombudsman, who ruled that summary information on the deal should have been released.
A draft copy of the agreement, which was carried out under delegated authority by the council, meaning it was not voted on by elected members, was leaked on social media during the hearing process.
The leaked document, dated September 2017, outlined financial assistance to be provided by the council to "the affected hapū of Matakarapa" Te Runanga O Raukawa Incorporated (TRoR) in return for the withdrawal of submissions to the Environment Court opposing the scheme.
The Court decision documents show withdrawals by Te Taiao O Ngāti Raukawa Environmental Unit and Te Roopu Taiao o Ngāti Whakatere on September 28 2017.
A statement provided to NZME by iwi said that in court documents, they had said they were principally against the discharge scheme "in its entirety".
"Pollution of human waste on to Matakarapa has effects on the environment, including effects on the Manawatū River, which in turn affect the iwi and their cultural and spiritual values," the statement said.
The statement said hui were called to oppose the scheme throughout 2016 as evidence was collected towards an April 2017 court appearance, however an adjournment was sought as legal advice indicated stopping the discharge scheme through those channels was unlikely.
"We opted to leave the court process with the view of developing a non-adversarial relationship with the council, to combat the negative effects of the planned works, with a long-term aspiration of finding alternative options," the statement said.
In May 2017 a hui attended by iwi representatives, former Horowhenua mayor Michael Feyen, councillors and Clapperton produced the current agreement.
Clapperton said it includes a clause that allows for legal action if the wastewater discharge proposal is "amended beyond scope or if HDC breaches its agreement".
Special buffer zones around areas of cultural significance have been created, with the council promising that no treated wastewater would be discharged in those zones indefinitely, Clapperton said.
TRoR chair Lindsay Poutama said the agreement acknowledges that Ngāti Raukawa is fundamentally opposed to the disposal of wastewater on Matakarapa.
"We are a pre-settlement iwi and this agreement will help us monitor the effects of this council project on our whenua," he said.
Jessica Kereama, Pou Taiao for Te Rūnanga o Raukawa said the project would help begin to resolve access issues to the ancestral land involved.
"We can...protect the areas where our tupuna [ancestors] rest," she said.
"We seek to preserve kōrero [verbal discussion] of our lives in these places and start to preserve our stories. These projects will have intergenerational benefits for our people and our shared history".
Iwi negotiation team member and recently elected Horowhenua district councillor Robert Ketu said it was important to remember future generations.
"It's about our future. You can become a victim or become part of a solution for the future," he said.
The arrangements for Matakarapa are expected to be overseen by a newly formed governance group consisting of chair Hemi Te Peeti and members Lani Ketu, Robert Ketu, Janelle Tamihana, Quentin McGregor, Johnny McGregor, Te Kenehi Teira and Heeni Collins.
The wastewater discharge project is due to begin in March next year.