NZ First wants maximum jail terms for synthetic drug dealers increased to 14 years, and National MP Simeon Brown is welcoming their support.

He's backing their proposed change to his Bill, which calls for penalties to increase from two years to eight.

Labour and the Greens say tougher penalties are the least effective approach.

But Brown is adamant penalties take suppliers off the street, while acknowledging the Government's harm reduction approach.

"We do need to do more to reduce the demand. We do need to provide more support for those who addicted to these substances. My argument is that it's not one or the other, we need to do both."

He said there's a mismatch in our legislation, where people who sell synthetic drugs can be jailed for up to two years.

"Whereas those who sell cannabis can get a maximum of eight years. That is demonstrably unfair, and Parliament has an opportunity to change that with my bill. I am calling on all parties to support that."