The fire which burned across half the Tiwai Peninsula in Southland has set back its regeneration by 50 years, the Department of Conservation says.
The blaze ripped through 930 hectares of land, destroying scrub, protected red tussock and other native plants. The area is also home to several species of unique invertebrates.
Four helicopters using monsoon buckets and, at the fire's peak, up to 60 firefighters tackled the blaze that began about 4.30pm on Friday. It took 24 hours to extinguish the fire.
DOC's Murihiku area manager Dave Taylor says the area had been regenerating for the last half century. Plants grow slowly in the harsh environment and it will take decades for flaxes and rare tussock grasses to recover, he says.
The fire was on public conservation land leased to the Tiwai Point aluminium smelter.
Mr Taylor says the fire seems to have been started accidentally from a tractor being used by contractors for the smelter company who were spraying weeds.
Smelter owner Rio Tinto has launched an investigation.
The Rural Fire Service says it was a particularly hot fire and the blaze also went underground so crews did a commendable job of stopping the blaze.
The smelter was not threatened as the flames were heading in the opposite direction to the plant.
At its height, the blaze could be clearly seen from Invercargill, about 20km away.