Uncertainty over what will happen to power prices - after the nation's biggest energy user cemented plans to close

Rio Tinto's aluminium smelter at Tiwai Point will wind up operations in August 2021.

The company cites high power costs and poor outlooks for the industry.

Energy Minister Megan Woods says the smelter uses 13 percent of the nation's power - so theoretically that electricity going back into the grid should lower prices.

However she says they also covered a lot of transmission costs - which will now have to be shared by other users.

Woods say they're working alongside Transpower to deal with a bottleneck in the grid - and let energy generated at Manapouri be moved around the country more easily.