Another investigation has begun into the death of prison guard Jason Palmer.
The 33-year-old was floored by a punch from an inmate while working at Waikato's Spring Hill prison on Saturday.
He died in hospital on Sunday after being taken off life-support.
A 21-year-old man has been charged with assault and remanded in custody.
Police and Corrections Department investigations are already underway and now the Labour Department has confirmed it is also probing the death.
The Labour Department investigation will run alongside the police inquiry and analyse the prison's health and safety systems.
The department expects the findings to be reviewed by Corrections officials, for their relevance to other prisons.
