Unlikely bedfellows the National Party and New Zealand First are working together to put peddlers of synthetic drugs behind bars for longer - possibly up to 14 years.

The Psychoactive Substances Amendment Bill, which would increase the maximum jail time for supplying synthetic drugs from two years to eight years, passed its second reading in Parliament last week.

National and New Zealand First supported it, while Labour, the Greens and Act opposed.

But NZ First justice spokesman Darroch Ball said his party's ongoing support depended on National backing an amendment to boost the maximum prison sentence to 14 years, in line with penalties for Class B drugs such as morphine, opium and ecstasy.

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"I would be very surprised if they didn't support it," Ball told the Herald.

The bill's sponsor, National MP Simeon Brown, said the caucus would have to discuss the 14-year prison penalty.

"There's certainly an argument that [suppliers] need to be taken seriously, so we're on the same page and we will discuss where to from here."

Law and order is an area where National and New Zealand First may have more mutual interests. They both opposed scrapping the three strikes law, which was one of Labour's election campaign promises.

About 45 deaths in the past year have been attributed to synthetic cannabis.

Labour and Green MPs all acknowledged the social destruction from synthetic drugs, but pointed to police evidence that increasing penalties would not make any difference.

Ball even agreed that the bill was not perfect and would do little to stop the harm from synthetic drugs.

"Will it stop all of those deaths? Will it stop all of those people from becoming addicted? Probably not," he said during the second reading.

"But that doesn't mean that we shouldn't be taking a hard stance on those manufacturers and those dealers."

He said the party's support for the bill was based on the possibility that it "will go perhaps just some way to deterring people from supplying this poison".

Green MP Chloe Swarbrick noted that the Greens opposed a 2014 decision to revoke interim licences that allowed the sale of regulated psychoactive substances.

"And what did we see?" she said during the second reading.

"We have seen the proliferation of psychoactive substances and their harm increase as a result of a lack of regulation. The chemicals have got nastier and cheaper to produce and throw together.

The bill offered nothing to reduce harm, she said.

"No one here is saying that drugs are cool or fun; what we are saying is that they exist and we have to deal with that. We have to reduce harm, and if we want to do something, why do we not do something that works?"

There was a heated exchange between Swarbrick and National MP Tim Macindoe after Swarbrick's speech, when Macindoe asked her how many lives had to end.

Swarbrick: "Don't you dare. Don't you dare."

Speaker: Order.

Macindoe: "How many young people have to suffer—

Swarbrick: "Before we do what works?"

Speaker: Order. Order.

Macindoe: "How many young people have to suffer the dreadful destruction—

Swarbrick: You're despicable.

Speaker: Order!