Undercover drug testing is again being carried out at Kiwi music festivals this summer.
More than one in three illegal drugs tested at festivals last year by non-profit group KnowYourStuffNZ were found to have contained unexpected substances.
Founder Wendy Allison said her group is operating in a "legal grey area" and is calling for greater clarity in the Misuse of Drugs Act.
"What we're doing reduces harm, we can prove that," Allison said.
"So what we'd like to see is a change to section 12 to create an exemption for harm reduction services, and that would allow us to operate completely legally and take ... the vulnerability away from the people organising the events."
As it stands, the law put drug testing volunteers at risk of being charged with possession, and also makes it illegal for someone to knowingly allow their premises to be used for drug taking.
Covert testing last summer found 20 per cent of drugs were completely different to what the user thought they were.
A further 11 per cent had additional ingredients in the drugs that they had bought.
"We provide drug-related harm reduction services ... drug checking which allows people to bring us samples of a substance that they may or may not intend to take, and we can test it for them and tell them whether it is or is not what they think it is," Allison said.
"There was very little we found that was not psychoactive in some way, so I would say that the risk would vary ... but they all carry some risk and some things we found were definitely potentially deadly."
Allison said of most concern from the last year's finding were cathinones, which had the street name of bath salts.
These were found at all festivals — and two of the cathinones found were new, and could not be identified.
"When you're getting into the field of substances that you don't actually know what they are, you can't even advise people of the risk," Allison said.
"All we can say is we don't know what this is, we suggest you don't take it."
Of the more than 300 samples tested by KnowYourStuffNZ at eight festivals, 39 distinct psychoactive substances were identified.
Just 76 per cent of those who thought they had bought MDMA had actually done so.
The group's call to update the Misuse of Drugs Act has the support of the NZ Drug Foundation.