A drug researcher is warning more fentanyl will arrive in New Zealand. 

The Government's working to tighten controls on 23 substances - helping to allow a police and Customs crackdown.

Health Minister Simeon Brown want to categorise three drugs similar to fentanyl as Class A substances, with harsher penalties for manufacture.

Massey University's Chris Wilkins says fentanyl is a cheaper alternative for many drugs.

"They've been replacing heroin with fentanyl - but now, increasingly, a whole lot of other drugs. And given that economic incentive, it's hard to see that New Zealand is going to avoid this problem."

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