Simon Bridges says National would be an ally in Donald Trump's War on Drugs.
At least 124 countries have signed on to the President's 'Global Call to Action on the World Drug Problem', which Mr Trump is promoting at the UN this week, but New Zealand has said no.
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The Opposition leader says he can't understand why Jacinda Ardern won't back the new drug war, calling it puzzling.
"Very strange. I really can't work out why the Coalition Government wouldn't sign up to this," he told RadioLIVE.
"It's American, yes, but it's very much under the auspices of the United Nations."
Mr Trump would require countries to develop national plans to cut demand for illegal drugs, stop supply at their borders and increase international cooperation. The policy has a special emphasis on synthetic drugs.
US media reports some of the 124 countries which have signed up have done so to avoid antagonising Mr Trump. But Ms Ardern says New Zealand won't be one of them, and a health approach is needed.
"No it's not our intention to and there are a number of other countries who haven't either," she says.
"We have an agenda that is focused on addressing issues around drug use. We have a number of challenges that are quite specific to New Zealand and the type of drugs that are present, but also I'm taking a health approach.
"We want to do what works, so we are using a strong evidence-base to do that."
But Mr Bridges says it should be a joint approach.
"Literally our best friends have, from the US and Canada to Britain, and to Australia, and more than that I think what we recognise it this is something with obviously serious impact," he says.
Ms Ardern has previously said the war against methamphetamine has failed, with the drugs now more accessible in New Zealand than cannabis.
"If we want to get to the issue, we actually have to look at what drives people's drug use in the first place," Ms Ardern told The AM Show in March.
"It's one thing to look at supply - dealing with people who are using solely through the criminal justice system hasn't worked. We do need to make sure we have proper services."