Before the law change volunteers worked in a "grey area" unsure whether they could face prosecution.
Know Your Stuff acts as a "last point of intervention" before attendees decide to take drugs.
"There's no fully safe drug use," Thompson said. "We tell people how to care for themselves, and not to mix drugs with alcohol. Alcohol pretty much shouldn't be mixed with any drugs.
"We are just trying to give underground peer-to-peer support."
The volunteers set up tents at festivals and advertise their open hours. A diverse range of attendees bring them drugs, which they test using an infrared spectrometer.
About 80 per cent of what is brought is MDMA, a party drug that induces euphoria.
An infrared spectrometer can pick out the chemical signature of substances. If the drug is not what the buyer thought, more than half of them will throw it away rather than taking it, Thompson said.
The service is free.
"People are overwhelmingly grateful," Thompson said.
Know Your Stuff has only two or three infrared spectrometers. Volunteers are completely unpaid, and donations are used to courier the spectrometers to festivals.
Each machine costs $50,000, and Thompson hopes the law change will increase donations and free up other organisations to take on drug testing.
"Drug use is huge in New Zealand and there's a lot of communities that we can't access," he said.
He got involved because he has been in and around the festival scene, and because Know Your Stuff has a good reputation. When it asked for help with volunteer co-ordination, he responded because he had experience in that field.
