Preliminary test results of the drug that hospitalised 12 people in Wairarapa during the weekend have come back positive for fentanyl.
This is the first time the drug has been discovered in New Zealand.
Police say the drug was sold as either cocaine or methamphetamine - it has a similar white powder appearance.
Detective inspector Blair MacDonald, National Drug Intelligence Bureau manager, said while these results are still preliminary, "it's important that we now know exactly what we're dealing with".
The analysis, by the Institute of Environmental Science and Research Limited (ESR), also indicated the sample tested contained the presence of mannitol or a similar sugar-type substance.