He has been found dead in the early hours of Wednesday, in circumstances described as involuntary homicide.
Public prosecutor Yves Dupas said in a statement that initial findings on the crime scene, in the village of Ouaco, are pointing to an initial assault from a 24-year-old man on a woman he was in a de facto relationship with.
Chief Boarat, 66, who was present at the scene, reportedly tried to stop the man from hitting his partner in their village residence.
The man, believed to be under the influence of alcohol, is then reported to have grabbed a wooden post and hit the chief on the head.
A medical team later found the old chief unconscious, with severe head wounds.
Attempts to revive him proved unsuccessful.
The suspect has been taken into custody, and investigations are ongoing.
He is facing charges of murder and assault against his de facto partner.
Witnesses are also being questioned as part of the inquest.
A post-mortem has been ordered to further establish the exact cause of the death.
The Boarat clan is the main chiefly entity of the Koumac area, which itself belongs to the chiefly area of Hoot ma Waap (one of the eight chiefly areas represented in New Caledonia's Customary Senate).