The Green Party is highlighting its policy to allow adults to grow and possess cannabis for personal use with new "420" campaign stickers.

The "420 Reasons to Vote Green" stickers come as the Green Party promotes itself as the most likely to push for meaningful drug law reforms.

National has ruled out decriminalising cannabis and Labour says some changes could be made but reform is not a priority.

Gareth Morgan's new Opportunities Party has announced policy to allow two cannabis plants per person for personal use.

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420 is a well-known term that refers to consumption of cannabis. Green MP Gareth Hughes posted a photo of the new stickers on Twitter today, at 4.20pm.

In December the Green Party said it wanted to allow adults to grow and possess cannabis for personal use, and ensure people using cannabis for medical reasons were not penalised.

A poll commissioned by the Drug Foundation last year found 64 per cent of respondents thought personal possession of a small amount of cannabis should be either legal or decriminalised.

Current penalties range from a $500 fine for possession to a 14-year jail term for its supply and manufacture. Cultivation of cannabis can result in a seven-year jail sentence.

Labour leader Andrew Little last year told a student radio station that his party might hold a referendum on cannabis reform, but later downplayed the comments.

Speaking at a drug symposium at Parliament last week, Labour's health spokesman David Clark said cannabis law reform was not a priority for the party.

United Future leader Peter Dunne, who oversees New Zealand's drug laws as Associate Health Minister, told the symposium he would move more quickly on reform if not for the reluctance of National.

He declared himself a strong supporter of Portugal's approach - where possession of up to 10 days' worth of low-risk drug supply results in a referral for health treatment, rather than legal consequences.

Debate about cannabis reform has been stirred by the cases of former union leader Helen Kelly and cricketer Martin Crowe, both of whom used the drug for medicinal pain relief before their deaths from cancer last year, and new approaches taken overseas including Australia.

Parliament is likely to next year vote on a member's bill by Green Party MP Julie Anne Genter that would go much further in freeing up supply of cannabis for medical use.

It would amend the Misuse of Drugs Act to make a specific exemption for any person with a qualifying medical condition to cultivate, possess or use the cannabis plant and/or cannabis products for therapeutic purposes, provided they have the support of a registered medical practitioner.

The law change has the backing of Labour, the Maori Party, Act and United Future. National is yet to decide how the party will vote on the legislation, or whether it will be a conscience vote.