A police officer and his colleague were “physically attacked by a group of around fifteen individuals” in Dumbea just outside the capital Noumea, forcing him to draw his weapon, said prosecutor Yves Dupas.
The total death toll from over a week of riots now stands at seven.
“In circumstances that have yet to be determined, the officer is said to have fired a shot from his service weapon to extricate himself from the physical altercation”, Dupas said in a statement.
“Initial findings show traces of blows to the officers’ faces,” the statement said.
The officer who fired the shots was taken into custody, the prosecutor said, adding that a probe into voluntary manslaughter by a person in authority was launched. Such legal moves are usually automatic in France when a policeman kills an individual.
The investigation will be conducted “with all the objectivity and impartiality necessary to establish the truth”, the prosecutor added.
The man was killed after Macron flew to the Pacific archipelago, located some 17,000 kilometres (10,600 miles) from mainland France, in an urgent bid to defuse a political crisis after more than a week of riots over voting reform.
Seven people, including two gendarmes, have now been killed since riots broke out on May 13.
But this was the first time that a civilian was killed by a member of law enforcement since the start of the violence.
France has enforced a state of emergency in New Caledonia, flying hundreds of police and military reinforcements to restore order.
