The assault at the Centre for Addiction and Mental Health, which left the nurse with a serious head injury, came just days after the provincial Labour Ministry charged the hospital in relation to a similar beating that occurred early last year.
The situation, the union said, can largely be blamed on a provincial funding freeze that has resulted in cuts to nursing and security staff.
"There's an escalation of the numbers of significant violent situations taking place in mental-health facilities across Ontario," Linda Haslam-Stroud, president of the Ontario Nurses Association, said in an interview.
"It's not just about staffing — but staffing is one of the issues that is starting to rear its head."
Given the ongoing investigation, CAMH said Friday it would be inappropriate to discuss the Dec. 29 incident, when it was reported to authorities, or why ministry investigators only showed up two days later.
"We hope to have more to share once the investigation is complete," hospital spokeswoman Kate Richards said in an email.
In August 2009, CAMH was fined $70,000 after pleading guilty to charges under the provincial Occupational Health and Safety Act in relation to two assaults on nursing staff in 2007 and 2008.
Just before Christmas, the facility was hit with new charges under the act in relation to last January's beating of two nurses.
In response to those charges, the hospital said it was "disappointed" and planned to defend against them in court.
Figures show CAMH staff reported 118 incidents in the past fiscal year that resulted in some form of harm. Four of the incidents were classified as severe and prompted the Labour Ministry to investigate.
Rani Srivastava, chief of nursing, said in a recent blog post that the facility is seeing more acutely ill patients at a time when staff are being urged to report all threats or similar issues. As a result, she said, the number of reported incidents has risen — not the number of violent assaults.
Haslam-Stroud was not impressed.
"CAMH will give any excuse in the world (but) that is unheard of in a health-care facility to have that many incidents," Haslam-Stroud said.
Other facilities in the province have also seen violent incidents in recent years.
For example, the Royal Ottawa Health Care Group has been fighting charges related to a 2012 case in which a patient allegedly choked and assaulted two nurses and a support worker.
In November, the Ontario Labour Relations Board ordered the health group to ensure round-the-clock security staffing at its facility in Brockville so nurses would have adequate protection when caring for or escorting violent patients.
Last month, an emergency room patient at Southlake Regional Health Centre in Newmarket, Ont., assaulted a nurse — the second such assault at the facility in two years.
"Why are we all of a sudden having all these additional increases in violent situations where nurses are being pummelled?" Haslam-Stroud said.
"When you talk about increased staff, it's not just nursing. It can be the whole mental-health team."
In a statement, Health Minister Eric Hoskins said the government would be reviewing the latest incident. He offered no further commitments.