The smelter at Tiwai Point produced a record amount of aluminium last year.
The smelter produced 354,030 saleable tonnes. The previous highest year was 352,976 tonnes in 2007.
The smelter is New Zealand's largest single user of electricity and creates export sales in excess of $1 billion.
The record production is despite generally low aluminium prices. Other smelters worldwide are cutting their production. The plant is up for sale by its global owner, Rio Tinto.
NZ Smelter general manager Ryan Cavanagh says the achievement is a significant milestone which the plant's employees can be very proud of.
He says the smelter is setting a benchmark for other industries in New Zealand.
Hard to repeat aluminium production level
The Tiwai Point aluminium smelter says it will struggle to better last year's record production level.
New Zealand Aluminium Smelters general manager Ryan Cavanagh says the secret to last year's record production is the plant's stable workforce, with almost a third of its 900 employees having 25 years' service.
Mr Cavanagh says the smelter makes the purest aluminium in the world so can grow production at a time when others are shutting down.
But, he says, it will be difficult to repeat the feat this year because the plant cut production last month by 5% due to high wholesale power prices.