"At approximately 9.08pm on March 17 ambulance requested police assistance at Hillmorton Hospital where an alleged assault had occurred, three staff had been assaulted by a patient," Hill said.
"Within three minutes of receiving the request, information was relayed to police that the alleged offender had been restrained and placed in a secure room.
"As the risk had therefore been reduced, police were soon diverted to a more urgent mental health incident in which a person had attempted suicide.
"Staff returned to Hillmorton at approximately 10.40pm to follow up on the alleged assault."
Hill said police were now waiting to take a full statement from the three victims.
Yesterday, John Miller of the New Zealand Nurses Organisation told Stuff the lack of police response raised "serious concerns" for staff safety.
"Our members are dealing with very unwell people. They absolutely need assurance that police [have] got their back if they need it."
Miller told Stuff he understood police felt they were called too often to help Hillmorton staff and the issue had been "brewing" in recent months.
"Nurses are trained in restraint. They will only call police when they absolutely need to."
Hill said police had to prioritise what jobs they attended based on a number of factors, such as; immediate risk to individuals, vulnerability of the people involved and potential for the incident to escalate.
"In this instance, our staff were confident that the situation at Hillmorton was
currently under control and a person's life was at risk requiring an immediate response."
He said a full and thorough investigation into the alleged assault is now underway and police are liaising closely with Canterbury DHB.
"We will be discussing this latest event to see what can be done jointly to prevent any more of their staff being injured at work."
CDHB mental health general manager Toni Gutschlag said he could not comment on individual patient or staffing matters.
"We have been open about the challenges currently being faced in our mental health service, and these have been well publicised," he said.
"Our staff continue to do an excellent job and we continue to support them in this."