The French-based organisation Observatory of Prisons says French Polynesia's main prison has refused to deliver mail to some inmates in apparent defiance of the law.
The organisation says it has been notified by the prisoners' support group Tamarii Nuutania which says the prison management refused to deliver a questionnaire about their conditions in jail.
It says the mail is being blocked on instructions of the French public prosecutor.
Tahiti's Nuutania jail is among the most overcrowded prisons run by France and the subject of numerous complaints.
Last year, Paris agreed to build a new prison for the territory.