Anti-drug campaigners insist testing at all concerts and festivals will save lives, but such testing is currently illegal.
KnowYourStuff NZ - a drug-harm reduction service that works in collaboration with the NZ Drug Foundation - found one in three samples tested last summer contained illicit substances.
But KnowYourStuff director Wendy Allison says section 12 of the Misuse of Drugs Act needs to be changed.
It prohibits organisers from acknowledging the existence of drugs at festivals, which means some are actually breaking the law by allowing Know Your Stuff to operate at events.
Ms Allison said her organisation was effective at promoting awareness around drug use and should be able to operate more openly.
"When we test people's pills and they turn out not to be what they think they are, more than half of them go on to change their mind about taking drugs," she told Newshub.
"I don't know any other single intervention that achieves that level of discouraging people from using drugs."
The organisation has been cleared to visit five festivals this summer.