Three nurses have ended up in the emergency room, after a patient in a drug-induced psychosis became violent at Christchurch's Hillmorton Hospital.

The incident happened on Saturday night.

Police took 90 minutes to arrive.

They're defending that time lag - saying within three minutes of the alleged assault, they were told the offender was restrained in a secure room.

Senior Sergeant Vaughn Lapslie says as the risk had been reduced, police were diverted to a more urgent mental health incident.