A drug-checking service is worried about how widespread a dangerous drug could be, now that it has been found in New Zealand for the first time.

Metonitazene is an opioid that has now been found in this country in the form of yellow powder.

Agencies warn it could also be found as unmarked yellow tablets.

Drug checking service Know Your Stuff deputy manager Jez Weston says it's too early to know how widely distributed metonitazene is within New Zealand.

But its arrival and availability on the black market is concerning, because even a small amount could cause an overdose of the strong sedative, and the drug has been responsible for deaths in the United States.

Know Your Stuff has received a strong response to an alert warning issued about the presence of metonitazene.

"We're hearing from people who are concerned, who are letting their friends know," Weston says.

"Word of mouth is often the best way of letting this kind of alert to really reach the people that it needs to reach - so that's working well."

He hopes the alert is early enough to prevent the drug from being distributed widely, as was the case when fentanyl was found.

"The issue with fentanyl in the Wairarapa a few months back, that put ... 13 people in hospital - that was a single import.

"I guess we can hope that this was the same kind of thing, it was potentially a single import [and] once the word gets out people won't take that or distribute that."

-RNZ.