Warnings about sinister substances being added to drugs in New Zealand has come as the summer festival season is about to begin.

For the past few years, KnowYourStuff NZ has been heading to concerts, where their volunteers examine drug samples so people can find out what's really in their substances.

Director Wendy Allison said they have discovered that some drugs in New Zealand are laced with potentially deadly substances.

"The thing that we found last year that was of most concern was cathinones. The street names for them is bath salts but we actually found a new one that has been causing trouble overseas, we found that at every event."

Testing last year showed one in three drugs had extra ingredients, or were a completely different substance to what the person expected.

"There was very little we found that was not psychoactive in some way, so I would say that the risks would vary between the things that we found and they all carried some risk and some things that we found were potentially very deadly," Allison said.

The group will be setting up shop at five festivals this summer.

But Allison wants a change to the Misuse of Drugs Act so festivals can openly allow drug testing at their event, something that is currently a legal grey area.

"If an event organiser gets us in to do harm reduction work, it means they are acknowledging they know people use drugs at their event, which makes them criminals. Which when you think about it it's a little bit silly because what we're doing reduces harm."

Allison's warnings come after Gisborne police announced they would be breath testing everyone who leaves Rhythm and Vines this year.