However, Public Service Association national secretary Erin Polaczuk said staff were entitled to do their jobs safely.
"We are concerned at this increase in assaults on hospital staff at Lakes DHB, regardless of the reasons given by the DHB's management.
"We are particularly concerned that half the reported assaults have taken place in Rotorua Hospital's mental health unit."
She said mental health workers across the country were under extreme pressure.
"The system is stretched to breaking point and we are increasingly concerned about the safety of workers."
Lakes District Health Board assistant communications officer Shan Tapsell said the health board believed the increased number of assaults reported was due to a new system which encouraged staff to report all incidents so it could better evaluate how to manage risk in the future.
She said the health board took every instance of violence and abuse seriously.
"There is zero tolerance to any behaviour which is intimidating, abusive, physically violent towards people or facilities or makes staff feel unsafe."
Ms Tapsell said many of the assaults happened because patients were very unwell.
"Sometimes they react to medicines or procedures in a way that they would not normally. Mental health patients are often very unwell when they come in to hospital and frequently misinterpret the engagements and actions of staff and other patients.
"Every assault is investigated and occasionally involves, where indicated, involvement of the police."
She said the hospital was trying to cope with assaults through more security and de-escalation training.
Serious assaults were reported to the police, but she said emergency department staff said police were rarely called for an assault on a staff member.
However, it was not uncommon for police to be called to help with violent or abusive patients.
She said recent refurbishment of the inpatient unit had given the ability to more intensely care for people who were acutely unwell in a different environment to those patients who were stable.
By the numbers
- 18 assaults on hospital staff in first five months of this year
- Half the assaults took place in the mental health unit
- In all last year there were 20 assaults