Meridian Energy says Rio Tinto's Pacific Aluminium unit wants to vary the terms of its electricity supply contract for the aluminium smelter at Bluff.
"Discussions are ongoing and remain confidential," Meridian said in a statement to the NZX.
The terms of the power supply contract for the Bluff smelter were renegotiated in 2007, with the new 18-year agreement to begin in January next year.
The smelter, operated by New Zealand Aluminium Smelters, is Meridian's biggest single customer.
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In its first-half accounts, Meridian described the contract as a "pricing agreement rather than a supply agreement".
The negotiations come as Rio Tinto looks to sell the Tiwai Point smelter along with 12 other aluminium producing assets in Australia.