It is "a miracle" no-one hospitalised with suspected fentanyl overdoses over the weekend died, NZ Drug Foundation boss Sarah Helm says.
Twelve people were hospitalised in the Wairarapa after consuming a white powder sold as cocaine and methamphetamine.
Preliminary testing has indicated the presence of fentanyl or a fentanyl-type substance.
The synthetic opioid is much more potent than morphine and is rare in New Zealand's underground drug market.
Helm told Breakfast the foundation had been "watching with horror" the harm fentanyl has caused overseas.
More than 60,000 people had lost their lives in North America last year due to a fentanyl overdose.
Just one gram of pure powdered fentanyl is the equivalent of 20,000 safe doses of the drug.
"Our concern has been the prospect of it entering our illicit drug market as adulterated methamphetamine or MDMA - which are slightly more popular drugs - and therefore the amount of overdose that that could cause quite rapidly in a New Zealand context," Helm said.
She said it was thanks to early warning system High Alert and drug checking that authorities knew about the hospitalisations.
High Alert is a notification system made up of health, law enforcement and non-Governmental organisations. Helm said High Alert would now enable authorities to "get on top" of the situation.
However, Helm said New Zealand was ultimately "under-prepared" for fentanyl's arrival.
"We cannot rest confidently that fentanyl will not enter into our drug supply again, if this was the end of it," Helm remarked.
She explained the country was "missing" naloxone - a medicine used to treat opioid overdose - and said it is "incredibly hard to access".
Helm said it was "a miracle" none of those hospitalised over the weekend had died. They had all been given - and responded well to - naloxone, but Helm warned others might not be so lucky.
"Overdose from fentanyl happens incredibly rapidly," she said. "Waiting for an ambulance can be the time that it takes for someone to die.
"We want this available."
Currently, a naloxone injection is prescription only and isn't free. People can buy an emergency kit containing two naloxone nasal sprays without a prescription, but it costs $92.