Rio Tinto has begun to restart some lost production at its Tiwai Point aluminium smelter at Bluff.
Rio shut one of its potlines last November after the failure of an electrical transformer.
The plant smelts 350,000 tonnes of aluminium each year.
The company says it will not be returning to full production just yet.
Global demand has slumped due to the economic downturn and an estimated 15% of total aluminium smelting capacity lies idle.