Police are investigating to determine the source of powdered fentanyl after 12 people were hospitalised in Wairarapa over 48 hours following the drug being sold as methamphetamine or cocaine.
This is the first time the drug has been discovered in New Zealand.
A warning has been issued by High Alert on Saturday night about an unknown white powder linked to multiple hospitalisations in Wairarapa.
Detective inspector Blair MacDonald, Manager National Drug Intelligence Bureau, said preliminary testing of the powder has indicated the presence of fentanyl, or a fentanyl-type substance.
He said police are investigating after the dozen people hospitalised are all believed to be linked to the consumption of the drug.
They displayed the same symptoms as an opioid overdose and at least six of them were unconscious and in serious condition when emergency services arrived.
The white powder has a very similar appearance to cocaine and authorities are warning it should not be consumed in any amount.
"The discovery of powdered fentanyl in New Zealand is of significant concern, due to the harm caused internationally by the synthetic opioid," MacDonald said.
"Just one gram of pure powdered fentanyl is the equivalent of 20,000 safe doses of the drug."
He said in North America last year, more than 60,000 people lost their lives due to a fentanyl overdose.
"We do not want to see that type of harm occurring in our communities," MacDonald said.
"Police are now working urgently to determine the source of the drug, and its prevalence in the community."
Anonymous reports of the drug can be made through High Alert's reporting system for unusual effects using the alert ID N22/029.