The Association for the Protection of Nature in New Caledonia believes the spill at Vale's nickel processing plant yesterday was a result of human error.
Operations have been suspended at the plant after 100,000 litres of effluent, containing some acid, ended up in a creek.
It is the fifth spill in five years at the Goro plant.
The president, Jean-Louis d'Auzon, says Vale insists it is working hard to improve its safety record and processes, but its actions do not reflect that.
"It seems it was acid with water thrown in the river, human error. Somebody has thrown the water into the river instead of throwing the water in the tank."
Jean-Louis d'Auzon says previous spills in the river indicate fish will die and plants will be affected.
He says the government of the Southern Province is in charge of monitoring the plant, but is he concerned the elections on Sunday will mean the spill will not get the attention it deserves.
Representatives from Vale and the government of the Southern Province could not be reached for comment.