E tū organiser Iriaka Rauhihi said the union was appalled by the second serious assault in just over a month at the hospital.
"What are they waiting for – a fatality?
"Assaults are frequent at this DHB and we're well aware of Allied Security's record in Christchurch as well. Our members feel unsafe and I'm not the only one worried that someone will die if things don't improve – our members are saying the same thing," she said.
Melinda Ch'ng, Waikato DHB director of business and support, confirmed the incident had happened on Tuesday after a guard was called to help with an agitated patient.
"The guard was struck in the face and received treatment in the hospital's Emergency Department and was discharged home," she said in a statement.
"Workplace violence from patients and family members towards staff is a recognised and ongoing risk; which is constantly being managed by the DHB. Any attack on staff or contractors working in our DHB is concerning. Every event is reviewed to see whether we can manage things differently."
The DHB and Allied Security had implemented initiatives to improve safety, including more training for security staff and extra "security resource", she said.
"The DHB has met with E tū to discuss improving hospital security."
The string of assaults had raised serious alarm bells, E tū campaign lead Mat Danaher said.
"We are now looking at a record of failure to stem the ongoing violence on hospital wards in Waikato and Christchurch."
The Herald has asked the Waikato DHB for comment.
DHBs were due to meet shortly with E tū to review hospital security, Danaher said - a move he said was long overdue.
"Violence on our hospital wards is a serious issue, affecting all staff. The nurses complain wards are unsafe and both they and our security members are frequently in the firing line.
"There are systemic failures including under-staffing, lack of training and poor health and safety processes. We are looking forward to the upcoming security review and welcome the fact that DHBs nationally are taking this issue seriously."