A homicide investigation is underway following the death of a man at the Auckland South Corrections Facility in Wiri on Thursday.
Det Insp Warrick Adkin, of Counties Manukau, said emergency services were notified at about 11:40am of a "serious incident" inside the men's prison.
"One man was found unresponsive and was worked on by medical staff, but sadly he has died at the scene.
"Another man has been injured and transported to hospital in a serious condition."
A spokesperson for Serco, the private company that runs the prison, said the second man is being treated for "non-life threatening injuries".
"The perpetrator has been secured, segregated and will be transferred to another prison."
The spokesperson confirmed the attack had taken place in the prison's kitchen.
Adkin said another person is assisting the police with investigations.
"Police remain at the site and we will be working with the Department of Corrections as part of our investigation," he said.
"We will look to issue further updates to media when we are able."
Serco's spokesperson said it will ensure "all evidence, including CCTV footage, is provided to police".
They added Serco will carry out a review into what happened and is providing support for staff and other prisoners.
"Assaults in prison are illegal, unacceptable and we do not tolerate them. Any perpetrators should expect to face criminal charges as well as prison disciplinary processes.
"All incidents are reported to the police. We will cooperate fully with all investigations into this incident," the spokesperson said.
Corrections Association president Floyd du Plessis earlier confirmed to Newshub a "serious incident" had taken place.
Du Plessis said lockdowns can usually last up to three hours, "but it depends on how widespread it is while the police cordon the area and do some initial investigation".
The prison can hold up to 960 men under an agreement with the Department of Corrections.
