Labour says an alleged assault at the Serco-run prison in Wiri is yet another sign the company is unfit to run New Zealand prisons and should go.
The alleged sexual assault took place on Thursday night.
The police and the Corrections Department are both investigating, and the Minister is aware of the allegations.
Labour's justice spokesperson Jacinda Ardern said the prisoner was moved out of segregation and locked in a cell overnight with another prisoner.
"Serco, for reasons that haven't become particularly clear, has put that prisoner temporarily into a mainstream prison and has obviously had dire consequences and they need to face up to the fact that they have once again breached their contract as a result."
The Corrections Department said the complaint was referred to police immediately.
In a statement, Corrections Northern Regional Commissioner Jeanette Burns said the prisoner was being supported and was being monitored in a health unit.
All prisoners had the right to be safe in custody, and Correction's contract with the prison operator clearly set a zero tolerance for anti-social behaviour, including sexual assaults, Ms Burns said.
Ombudsman receives 19 complaints against Serco prisons
Meanwhile, the Ombudsman has revealed 19 complaints have been made to its office since January over the Mount Eden Corrections Facility.
Corrections was now overseeing the prison while an official investigation is carried out into allegations of mismanagement by its private operator Serco.
The Ombudsman said four of the complaints involved allegations of assault and remained under active consideration.
Twelve complaints have been completed and dealt with by various methods, including the matters being referred to Corrections or the Health and Disability Commissioner.
Earlier this week Radio New Zealand reported allegations of other assaults at the Wiri prison.
The Ombudsman has received four complaints against Auckland South Corrections Facility, three relating to property issues and another to a prisoner's transfer.