A report into an escape from Mt Eden Prison in Auckland says officers ignored a warning alarm for nine minutes before investigating.
In a brazen escape in May, Aaron Stephen Forden used knotted bed sheets to climb down the prison wall and was at large for three days before being recaptured.
A report by the Department of Corrections, obtained under the Official Information Act, says control room staff failed to properly monitor the prison's security systems.
An audible alarm sounded when the prisoner escaped, but it was deactivated and ignored for nine minutes.
A prison officer walking to work saw a rope hanging from the walls and raised the alert using his mobile phone.
The prison guard's union describes the report as a whitewash. President Beven Hanlon says no one was available to investigate the alarm at the time.
Mr Hanlon says prison officers also need to receive better training on monitoring alarms.
The report acknowledges that Mt Eden was not adequately staffed, but says this did not contribute directly to the escape.
The department says it has improved a number of systems as a result of the investigation.