Police say at least seven deaths in the city this month have been linked to the use of the illegal drug.
The chief coroner and police issued a warning about synthetic cannabis use last week. Ambulance service St John, meanwhile, has said it attends about 20 life-threatening cases linked to the drug each day in Auckland.
Over the weekend, nine people suspected to have taken synthetic cannabis were seen at Auckland City Hospital's emergency department. Each suffered seizures or collapsed.
It's had 31 similar cases over the past week and one death.
Chief Medical Officer Margaret Wilsher said people taking synthetic cannabis should realise they were putting their life at risk.
Symptoms included agitation, psychosis, an increased heart rate, seizures, and sometimes collapse, she said.
City Missioner Chris Farrelly said some of those symptoms could be seen in their clients, a large proportion of whom face addiction issues.
"What is most predominant for us is just an increase in aggression in people who are normally quite placid and quiet. [They are] almost out-of-character violent issues and violent episodes."
It had been a trend happening over months, but there had been a surge in the past few weeks, he said - which led his organisation to believe the substance was being spiked with something else.
"There's real concern with what it's being spiked with. Some substances that are very, very dangerous and very poisonous with some catastrophic side effects."
He said staff at its medical, detox, outreach, and drop-in centres were seeing the violent behaviour almost daily and staff were now being trained in how to handle unpredictable violence.
A police spokesperson said they were aware there of some homemade varieties of synthetic cannabis available, but police testing so far had not flagged anything more out-of-the-ordinary than the usual ingredients.
There had been no traces of household chemicals or horse tranquilliser, she said.
The city mission was working closely with district health boards and police to address the issue, Mr Farrelly said.
Prime Minister urges responsibility over synthetic cannabis use
Prime Minister Bill English said it was essential synthetic cannabis users start taking some personal responsibility if they want to stay alive.
"The most important thing is that people that are considering taking recreational synthetic cannabis have to be aware that it could kill them," Mr English said.
"Don't take the substances, we don't know what's in them."
Mr English said the spate of deaths hadn't changed his view that natural marijuana should remain illegal.