Footage of gang fights filmed in Auckland's Mount Eden prison are being shared on social media.
Several videos depicting violent scuffles between inmates have been filmed using smuggled mobile phones and uploaded online.
Warning: the video below contains graphic images which may be disturbing to some viewers
The Department of Corrections said it was working alongside Serco, the company managing the prison, to investigate how the footage was taken and if those involved will face charges.
It said while prisoner-on-prisoner assault could not be completely prevented, it was reviewing whether the safety of prisoners was compromised by management.
Serco Director of Operations Scott McNairn said additional security measures had been put in place, including extra searches and limited access to exercise yards.
Mr McNairn said the smuggling of cellphones into the prison was a real issue, with 27 mobile phones and 22 SIM cards confiscated this year alone.