Sometimes substances that have different effects, or drugs that are easier to overdose on, can show up.
KnowYourStuffNZ tested 330 samples of people's illegal drugs and found 31 per cent of the samples were not entirely what people expected.
Of those who had adulterated drugs, 63 per cent - almost two thirds - decided to ditch the drugs when they learned their actual contents.
Spokeswoman Wendy Allison said: "We run this service so that people can make informed choices about their drug use, because that information could keep them alive."
In the 330 tested samples, 39 distinct psychoactive substances were identified.
Festival-goers believed half the samples offered for testing were MDMA, but of these only 76 per cent were.
About a quarter of all drugs offered for testing were thought to be LSD, with 89 per cent of those testing positive for the psychedelic drug.
Lower levels of other drugs such as cocaine, cathinones, amphetamines and ketamine were also detected.
"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," Allison said.
"So, they take a massive additional risk on something we now have the technology to address."
Forty-four per cent of New Zealand adults will try an illicit drug in their lifetime, according to Ministry of Health figures. But the nature of the black market means it's almost impossible to know what is in the drug without testing.
Drug checking is commonplace at music festivals in Europe. The Dutch government has run an official drug testing service since the 1990s, while Austrian checking services run in a legal grey area, where the police don't hassle users.
Eight festivals in the UK are expected to offer drug testing services this summer, including Reading and Leeds festivals.
Overdose deaths, while rare, can occur. Three people died earlier this year in Melbourne and more than 20 were hospitalised in one weekend after a suspected bad batch of MDMA.
Currently, drug checking was a "legal grey area" here too, said Bell.
The Misuse of Drugs Act says it's illegal for anyone to knowingly allow their premises to be used for drug taking. The maximum penalty is 10 years in prison for a Class A drug.
But Bell said a more likely sanction here was for a festival to lose its liquor licence or permit to use local body-owned land - preventing organisers from running the festival.
"Festival organisers are clamouring for drug checking services but are fearful of the legal risks, Bell said.
"Their worries are about losing their ability to run the festival.
"We need to remove any legal barrier to these life-saving services.
"Ultimately, we think that not in the too distant future, it will be expected that these are at music festivals, and I can see a future where these things are mandatory at music festivals, as part of their wider health and safety plan," said Bell.
"Drugs have changed, science has moved on, and new technology has been developed. It's time government played catch up."
Bell said even police were "interested in the concept" and wanted to be kept informed of any progress he made on the issue.
"Such a partnership would be appropriate and beneficial to the New Zealand Police purpose of 'Be Safe, Feel Safe'," Detective Inspector Virginia Le Bas, police national operations manager organised crime, said in a letter to Bell.
Dunne said it was clear recreational drug testing was a good idea, and the Government would look at the legality of the issue.
"We should. We should, absolutely," Dunne said.
"I think that's something we'll clarify once we review the Misuse of Drugs Act, which is scheduled to occur in the next 18 months or two years.
"I can't see [drug testing] causes any particular problem. I think they actually provide a benefit to people, and I think it is a common sense solution.
"I've certainly had a number of discussions with the Drug Foundation and with a number of individual MPs, I think there's a general sense that this just makes sense and is a logical thing to be doing... I think we should do it."
