Two inmates were assaulted with makeshift weapons in separate incidents at Auckland Prison on Sunday.
Prison director Andy Langley said the assaults appeared to be gang-related, and both involved maximum-security prisoners.
Both prisoners were in a stable condition after being taken to hospital.
Mr Langley said both incidents were brought under control immediately, and the perpetrators had been secured.
He said several of the people involved would be placed in segregation for safety reasons.
"Any prisoner who resorts to violence will be held to account."
Mr Langley commended staff for responding with "bravery and professionalism".
He said more than 75 percent of the country's prison population had convictions for violence, and prison assaults were disproportionately committed by gang members.
A police spokeswoman said police were investigating the incidents, according to Stuff.
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