And a new law to ban the substances is having no effect, she said.
Assaults on Otago's Southern District Health Board mental-health services staff in the first seven months of the year were up about 37% compared with last year, figures show.
Overall, assaults on Otago health services staff were up about 36% on last year.
Synthetic cannabis use was more prevalent than alcohol or illicit drug use among some mental-health services patients, Mrs Casey said.
Those using the synthetic substances were likely to be emotionally unstable and lack inhibition.
"It's a huge problem," she said.
She could not quantify how much of the increase in assaults was due to synthetic cannabis.
That would require detailed analysis of casework, she said.
However, there was a definite rise in incidents associated with the substances in the past year or so.
"It is having a significant impact on wards," she said.
A move to reduce harm associated with synthetic cannabis use, through temporary bans on 28 substances pending legislation to deal with the legal highs industry, had made no visible difference. Synthetic cannabis was available if people wanted it, whether on the internet, or in the community, Mrs Casey said.
"All we can do is advise people not to use it."
Some inpatients on day leave got synthetic cannabis before returning to the wards, she said.
Legal highs were used by teens and those in their 20s and 30s.
The effects of synthetic cannabis were worse than the effects of cannabis itself, she said.
In the emergency psychiatric service, alcohol was the biggest factor in assaults, she said.
Mrs Casey believed the reported rise in assaults was also because of increased awareness and reporting. Staff were encouraged to identify what they considered an assault - including verbal attack - rather than using health board definitions.
Where appropriate, assaults were referred to police, who decided whether to prosecute.
In general, society was becoming more violent, and aggressive, and that was feeding through to health services, she said.
In the first seven months of this year in Otago there were 131 assaults in mental-health services: seven in the emergency department, one in intensive care, four in medical, two in support services and one in food services.
National Poisons Centre toxicologist Dr Leo Schep said there had been a surprising rise in notifications after a quiet period.
Although he could not be certain, recent notifications indicated a more "potent" product might have hit the streets, which could be more likely to cause seizures and psychosis.
World-first legislation due to become law next year would hopefully solve the problem.
New Zealand Nurses Organisation professional services manager Susanne Trim said that as drugs and alcohol use increased, so did the risks to health staff dealing with violent or disturbed patients and members of the public.
A Ministry of Health spokesman said it did not collate nationwide data on the problem.
