National grid operator Transpower says the hydro lake levels are at 67 percent of where they would normally be at this time of year, but there is no cause for worry yet.
The Manapōuri catchment area has had just 77 percent of its normal rainfall so far this year.
Photo: 123rf
The smelter consumes about 13 percent of the country's power and it will reduce its hourly use by up to 30.5MW.
A spokesperson said the decision would ensure there was enough supply for the rest of New Zealand.
"We are pleased to now be in position to be able to do more to assist New Zealand's security of supply for all customers should the lake levels not be replenished in the short term."