Rio announced last year the smelter would close by the middle of 2021.
The company said today the extension provided certainty to the smelter's 1000 employees, the local community and customers, while providing more time for all stakeholders to plan for the future.
Rio said the new agreement with Meridian would make the smelter economically viable and competitive over the next four years.
Discussions continue with the NZ government over the smelter's high transmission costs.
The extension also provides time for detailed closure studies to be completed and for NZAS to support the government and Southland community in planning for the future.
"We are pleased to have reached an agreement with Meridian Energy that will enable the Tiwai Point smelter to continue producing some of the lowest carbon aluminium in the world," Rio Tinto Aluminium chief executive Alf Barrios said.
"This agreement improves Tiwai Point's competitive position and secures the extension of operation to December 2024.
"It also provides Rio Tinto, the New Zealand government, Meridian, and the Southland community more time to plan for the future and importantly gives our hard-working team at Tiwai and our customers the certainty they deserve."
NZAS is a joint venture between Rio Tinto (79.36%) and Sumitomo Chemical Company (20.64%).