Charges had yet to be laid and it was unclear when they would be, the spokesperson said.
Mr Massingham said Corrections had a zero tolerance policy toward prisoner assaults on staff and other prisoners.
"No assault is acceptable, however, it is an unfortunate reality that from time to time this will occur. We manage some of New Zealand's most difficult and challenging citizens. Therefore, violence can be a risk as many offenders resort to this behaviour as a means of resolving issues and of expressing themselves," he said.
"Understanding and managing this risk is a challenge common to all correctional jurisdictions."
Hawke's Bay Regional Prison, on Mangaroa Rd, southwest of Hastings, has capacity to house up to 682 minimum to high-security male inmates.
In January an inmate died at the prison after staff found him in his cell in an unresponsive state and were unable to revive him.
Police said at the time of the death that they did not consider it suspicious and had referred it to the Coroner.
Prior to that, the last death in custody associated with Hawke's Bay Regional Prison was in 2011 when a 54-year-old inmate died in a Department of Corrections van while being transferred to a forensic unit in Wellington.