The prisoner was placed on directed segregation and will face internal charges.
The incident will also be referred to police.
Pearse said the fire had been contained to the prisoner’s cell.
“There was no structural damage to the prison and there was no threat to the wider security of the prison.”
Fire and Emergency NZ shift manager Murray Dunbar said two Whanganui fire crews spent about an hour extinguishing the fire at the prison.
Department of Corrections said for operational security reasons they were not able to provide further information on how the prisoner started the fire.
“Safety is our top priority and staff respond immediately when smoke is detected.”
Eva de Jong is a reporter for the Whanganui Chronicle covering health stories and general news. She began as a reporter in 2023.