The Corrections Association wants bars installed on windows in all low-security prisons.
This follows a series of attacks on prison guards, the latest of which occurred last month at Waikeria Prison in Waikato when two guards were assaulted by two prisoners who got out of their cells.
In another incident in Hawke's Bay, a guard was disfigured when boiling water was thrown at him by an inmate in June.
The Corrections Department is currently undertaking a nationwide prison safety review.
Union president Beven Hanlon, says the guards at Waikeria have suggested bars be put on windows to improve security.
He says the review should also check that prisoners are being classified correctly.