A Rotorua security expert wants synthetic cannabis to be banned, blaming the drug for a sharp increase in assaults on his staff.

Brett Wilson, whose Watchdog Security firm attends noise-control complaints and patrols in downtown Rotorua at night, said before psychoactive substances were around there was only one assault every two years.

In the past year there had been four and all had resulted in police arrests.

One incident had involved a group of young women and he said his staff member had to defend himself.

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"We've had nothing too major in terms of injuries [to my staff] but the assaults were unprovoked," Mr Wilson said.

"One involved three girls and a guy going nuts and attacking one member of staff. They were capable of looking after themselves but he had to hit one of the girls. [These drugs] have potential to cause harm. The boys were able to deal with the situation but they were unnecessarily aggressive. I think synthetic cannabis makes them more liable to lash out. We've noticed increases in aggression, particularly if they're mixing it with alcohol."

He said police had told him those who assaulted his staff had been using psychoactive substances.

Mr Wilson wants council to ban the sale of the drug in the city.

"In my opinion the law doesn't go far enough. No positives come out from the availability of synthetic cannabis.

"There are no natural substances in it and your brain's not designed to cope with synthetic chemicals. I don't want it available in our city." Rotorua Mayor Steve Chadwick said one of council's first priorities for the year would be to decide on its local authority policy on the sale of psychoactive substances. That policy could determine where in the city shops could sell the drugs.

"We hate this stuff and if we have a policy we're almost condoning it," she said.

"We're caught between a rock and a hard place. We want to know what is the strongest [wording] we can put into our policy. One of the biggest feedbacks we've had is don't call them legal highs.

"By saying legal highs you're saying they're safe for you and they're anything but."

Acting area commander of Rotorua police Inspector Tim Anderson said police would support any controls the council could place on the sale of synthetic cannabis if it would "reduce consumption of it".

"The good thing is it's not allowed to be sold out of your corner dairies any more. We've already prosecuted a store for supply to an under-age customer - just like alcohol we run controlled-purchase operations.

"We're also noticing people who have existing mental health conditions using synthetic cannabis and it's aggravating their condition," he said.