Three investigations are under way after Auckland Prison staff were caught on CCTV footage using potentially excessive force against inmates.
Last month, a prison staff member was briefly hospitalised with stab wounds after an attack by three inmates using makeshift weapons.
The inmates were also injured, with two also needing hospital treatment.
Auckland Prison director Andy Langley said two staff members have since been suspended, and a third was still being spoken to.
Corrections Northern Regional Commissioner Jeanette Burns said the incident was being treated very seriously.
"The prison director who actually did the initial review was sufficiently worried about some of the injuries that were sustained ... that he referred it to police and now that's become a police investigation."
She said an internal review was being carried out and prison inspector Louise MacDonald had also been called in.
"The inspectorate will come in and review footage, talk to prisoners, talk to Corrections officers and just do a thorough review of the situation and what events lead up to the incident ... and what happened afterwards."
Ms MacDonald will also look at any connected incidents and whether the management of the inmates after the incident was done in a safe, fair and humane manner.
Ms Burns would not go into detail about what kind of "potentially excessive force" was shown in the CCTV footage.
"What I do want to say is it was picked up as part of a regular review. Any use of force that is used in a prison is always reviewed."
A Safer Auckland Prison Plan is also being pulled together to improve conditions for staff and inmates.
"Again, that's just good practice after any really serious incident. So we're looking at what other things we can do to make things a bit safer in there.
"We are dealing with the country's most violent prisoners ... in that particular prison."
A draft report will be completed by 29 June.