Production at the aluminium smelter in Bluff is down by the equivalent of 280 tonnes a day because of equipment failure.
Owner Rio Tinto says an electrical transformer failed late on Sunday, causing one of the potlines to be shut down.
The company said on Monday that it was assessing the degree of damage to the transformer and it is too early to determine the duration of the curtailment.
Lost production is estimated at 8,500 tonnes on a monthly basis.
The smelter was already operating at a reduced rate of 315,000 tonnes annually before the incident.
It has been running below capacity since May to help reduce demand on the national electricity grid.
Production capacity at the Bluff smelter is 350,000 tonnes of aluminium annually.