More than one-third of illegal drugs tested at Kiwi music festivals last year contained unexpected substances, prompting calls to make the covert tests legal.
Non-profit group KnowYourStuffNZ ran undercover tests on more than 300 party drugs at eight festivals, finding 31 percent were not exactly the drug festival goers thought they had purchased.
They also found 20 percent of drugs were entirely different to what a user thought they had purchased with cathinone, a common substitute for the substance claimed to be in the drug.
This led 63 percent of people to reconsider whether they would still take the drug or not.
KnowYourStuffNZ spokeswoman Wendy Allison says the tests were run without the knowledge of police and allowed festival goers to check the ingredients in drugs they had bought.
"We run this service so that people can make informed choices about their drug use, because that information could keep them alive," she said.
"Without drug checking, people go to events and use drugs, which they purchase illicitly with no assurances that what they have is what they think it is, or what quality and strength it is."
Associate Health Minister Peter Dunne personally backed the testing because it reduced harm and was a "common sense" approach.
However, drug checking remains a legal grey area under the outdated Misuse of Drugs Act, developed for the 1970s, according to the NZ Drug Foundation executive director Ross Bell.
He said the Act put drug testing volunteers at risk of being charged with possession, while the law also makes it illegal for someone to knowingly allow their premises to be used for drug taking.
This meant festival organisers were "clamouring" for drug checking services, but were fearful of the legal risks, he said.
Mr Dunne told TVNZ on Sunday the fact drug checking was against the law was a "legal anomaly" that he thought would change soon.
"It is inevitable that when the Misuse of Drugs Act is reviewed in the next couple of years that this change will be considered," he said.
"My own personal view is I am in favour of it.
"It is a preventative measure, it is important people have a public safety message."
Thirty nine distinct psychoactive substances were identified among the more than 300 samples tested by KnowYourStuff NZ, while only 76 percent of those people who thought they had bought MDMA had actually done so.