"Within three minutes, information was relayed that the alleged offender had been restrained and placed in a secure room," he said.
"As the risk had therefore been reduced, police were soon diverted to a more urgent mental health incident.
"Staff returned to Hillmorton around 90 minutes later to follow up on the alleged assault."
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."
A recent rise in patients suffering methamphetamine-induced psychosis had led to more violence in the mental health inpatient units, Miller said.
He said the union was putting together a list of concerns and potential solutions to present to the Canterbury District Health Board (CDHB) this month.
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."
A health and safety review was put in place at Hillmorton Hospital last October after earlier reports of staff being bitten, hit, and injured.
Between January and October last year there were 11 ACC workplace injury claims for staff working in the assessment, treatment and rehabilitation unit.
Four people were also injured after a patient lit a fire at the hospital in December.
The Herald has approached the Nurses Organisation for comment.
