Mighty River Power and the Tuwharetoa Maori Trust Board have signed an agreement over water in Lake Taupo, a deal whose terms the electricity generator says are secret.

The agreement is for a period of 52 years and provides an "enduring set of arrangements" in relation to the operation of the lake.

Mighty River chief executive Fraser Whineray said the commercial deal was confidential but did not impose any new restrictions on how his company could use water from the lake for its Waikato River hydro system.

"We're not disclosing the key commercial terms (but) an operating easement will be registered in on the Lake Taupo title."

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He said the length of the agreement extended well beyond the current resource consent which expires in 2041.

"It won't detract from any of the operating parameters for the Waikato hydro scheme."

Lake Taupo is the primary source of Mighty River's hydro hydro scheme, which accounts for around 10 per cent of the country's electricity supply. The hydro storage also benefits downstream users of water, including Hamilton and Auckland drinking water.

In the leadup to the MRP's partial listing in 2013 there was speculation the company would have to pay for the right to store water in the lake.

Tuwharetoa was reported then as previously happy for Mighty River Power to store water on their land free of charge when it was fully owned by the taxpayer but the sale of 49 per cent of the company had led to a change of heart.

A MRP spokeswoman said the settlement was below the threshold for a material impact on the company which would need to be disclosed to the NZX.

Chairman of the Tuwharetoa trust board, John Bishara, said the agreement would be beneficial to all involved.

"This positive outcome reflects the mana Tuwharetoa holds as owners of the Lake Taupo lakebed and we look forward to working further with Mighty River Power. As far as the recreational and other lake users are concerned nothing changes." he said.

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Tuwharetoa trust chief executive, Topia Rameka said the trust outlined the agreement at its annual meeting during the weekend and it was universally supported.

"We have a strong working relationship with Mighty River Power and we look forward to exploring further potential operations. We've built successful ventures before and this is an exciting opportunity," he said.

Mighty River Power shares were trading down 2c at $2.97 this afternoon.