A drug counsellor says the effects of synthetic cannabis can be worse than meth, with users kept up for days and sometimes being driven into psychosis.
Eight people have died so far this month, the latest a 24-year-old man who died yesterday after becoming seriously ill from smoking the drug.
Clinical director of Alcohol & Drug Assessment & Counselling (ADAC) Roger Brooking said the drug was far stronger than most users realised.
"It tends to keep them awake for days on end, much like methamphetamine does.
"My experience would be that it drives people psychotic, or at least in that direction, more quickly than methamphetamine.
"It's a lot more addictive than the plant cannabis, it has no business being called cannabis.
"Normal cannabis, it's kind of psychologically addictive, but not physically addictive.
"But the synthetic chemicals being used seem to be a lot more addictive, and once people start they find it very hard to stop."
Brooking believed there needed to be big changes to stop the problem getting any worse, including decriminalising cannabis.
"I mean, if I was in charge of this I'd decriminalise all illicit drugs, as they have in Portugal.
"Because these drugs are illegal, when people use them and get caught they get steered into the justice system instead of getting steered into the health system.
"For the average user, these cause health problems rather than legal problems.
"If cannabis was decriminalised, people wouldn't have to go looking for some of these other substances."
Associate Health Minister Peter Dunne agreed that a "significant part" of problem was that synthetic cannabis was banned, and then driven underground.
He said he had spoken to Prime Minister Bill English about the idea of regulating the drug, and would "keep the discussion going".
"Had we had a regulated market in place, this stuff would have had to be submitted for testing before being sold.
"Because its been driven underground we don't know what it is, it's not being tested, and we're dealing with consequences. We're playing catch-up all over again.
"There are so many new psychoactive substances coming down the pipeline, whether this is a blip or the start of a flood, we just don't know."
Dunne said that the current deaths seemed to be part of a "serious outbreak", but it was difficult to get accurate information about what the substances actually were, and who was selling them.
Matt Bowden, who was once seen as the "godfather" of New Zealand's legal high industry, moved to Thailand in 2015 after many drugs were banned under the Psychoactive Substances Amendment Act 2013.
He told The Wireless that the blanket ban of synthetics and all animal testing didn't work, and the law had created preventable deaths.
"We know there are millions of different molecules which act at the cannabis receptors.
"But as all the safer bets and more benign molecules were banned, the more dangerous ones came to the fore and eventually the dreaded day arrived when some quite lethal compounds hit the market."
Bowden told The Wireless politicians needed to regulate natural cannabis.
Synthetic cannabis is manufactured by spraying a chemical mixture onto finely chopped plant material, which is then left to dry before packaging. ESR said that because there was no quality control in the process there could be big variations in which chemicals, and how much of them, was present.
Synthetic cannabinoids are themselves likely to be harmful. The chemicals are so new that their medical effects are unknown, or only anecdotal.
The synthetic compounds are much stronger than the cannabis plant. One synthetic cannabinoid, AMB-Fubinaca, is reported to be 75 times more powerful than the THC in cannabis plants.
The drugs could also be contaminated. Eastern Districts Police said sellers have admitted to adding kitchen spray cleaner, rat poison, and horse tranquiliser.