The USTKE union in New Caledonia has criticised the conditions and management of the Noumea prison.
A union leader says 320 inmates are being held in a facility built to house only 180, with cases of 5 prisoners sharing a nine square meter cell in searing summer heat.
He says minors are being held with other inmates in a complex built more than a century ago.
The union also says 90 percent of inmates are Kanaks but the jail is run from Paris while few of the wardens are drawn from the indigenous population.
A new jail is to be built in New Caledonia but its site and its funding are yet to be decided.