If the public votes in favour of legalising cannabis in a referendum, a National-led government would change the law accordingly, National Party leader Simon Bridges says.

Photo: RNZ / Richard Tindiller

A referendum on legalising the personal use of cannabis is part of the confidence and supply agreement between the Greens and Labour parties.

Under the deal, the referendum will be held at or before the 2020 general election.

Justice Minister Andrew Little said the government would decide in the next couple of months whether the upcoming referendum would be binding.

"One of the decisions the government is going to make is whether the it's a binding referendum ... in order for a binding referendum to take place there has to be a reasonable degree of specificity and certainty about what would follow a 'yes' vote."

Mr Bridges said National would enact a law change if that was what New Zealanders wanted.

"Oh I think we've got to, I mean we've got to go with what the people want and what a referendum tells us.

"We've got a bit of water to go under the bridge, we've got to see the question, we're going to have an informed debate I hope on the issues, but absolutely on principle we support referendums and their outcomes."

When asked before she went on maternity leave, Prime Minister Jacinda Ardern would not yet commit to whether Labour would legalise marijuana if a referendum favoured the change.

Mr Bridges also said he would support a law change to legalise euthanasia if a public referendum came back in support.

Asked whether cannabis should be legalised to undermine the synthetic cannabis trade, Mr Bridges was not keen, saying two wrongs did not make a right.

Health Minister David Clark said legalising marijuana was not necessarily the answer to the synthetic cannabis crisis.

"With marijuana we know that it does cause harm in adolescents, and there are a number of mental health conditions that are affected by cannabis use, so we tread carefully in all of these things."

Dr Clark said urgent advice was being sought on how to deal with synthetic cannabis and the spike in deaths from the drug.