Vale in New Caledonia says the discharge of solution from its Goro nickel processing plant has stopped after 100,000 litres of effluent, containing some acid, ended up in a creek.
In a statement, the company says the spill happened at two o'clock yesterday afternoon, shortly before the authorities were notified.
It says the spill is due to a misconfiguration of the circuits transferring solution between two retention ponds.
Vale says response teams have helped limit the impact by pumping the ponds, and the water quality of the creek is now back to normal.
It says some fish were found dead in the creek.
In February, five New Caledonian environmental groups won nearly half a million US dollars in compensation from Vale over a major acid spill at its plant five years ago.