Andrew Little has revealed why the Government proposed the legal age of 20 in its draft legislation on recreational cannabis law reform.
Little, the Minister of Justice, said other countries that have legalised recreational cannabis in the past were looked at when deciding on a proposed legal age.
"We know that the medical science says that the human brain is still developing up to the age of 25, but if you put too high an age on it you just encourage the black market."
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National's drug law reform spokesperson Paula Bennett said she's "not sure about the age", admitting that it's "a bit unrealistic to expect" people not to try cannabis until they're 25.
But she said while the Government seems "genuinely concerned" about reducing the harm caused by cannabis consumption, there should have been public submissions on age.
In Canada, people aged 18 or older (in some provinces 19) can possess up to 30 grams of cannabis, after the country's Parliament passed the Cannabis Act in June last year leading to recreational cannabis legalisation.
The Government announced on Tuesday that New Zealanders will vote on legislation to legalise recreational cannabis at the 2020 election.
Little said one of the objectives of the draft legislation - which is yet to be released - would be trying to get rid of the cannabis black market - a similar purpose Canada had.
"The choice that New Zealand voters will have is: is the status quo a better way of controlling what is a harmful substance or is legalisation and allowing open and transparent regulation control a better way?" Little asked.
The Government's three parties - Labour, New Zealand First and the Greens - reached a consensus "some weeks ago" about how the referendum would proceed, Little said. It was endorsed by Cabinet on Monday.
"People will know exactly what a yes vote will entail, so all the questions that could possibly be raised about what regulation of what cannabis would look like will be able to be answered."
The New Zealand Drug Foundation executive director Ross Bell said there are arguments on either side for setting an age of 20. He said people 19 and below would instead have to access cannabis from the criminal black market.
But he also argued that the science around the impact of alcohol on developing brains is even stronger than cannabis, and yet the legal age for alcohol consumption is 18.
Is the referendum binding?
The Government has been criticised over its use of the word "binding", since New Zealanders will be voting on legislation that will then need to be passed in Parliament.
Little promised in December last year that the referendum would be "binding", and on Tuesday defended that claim, insisting it was the right word to use.
"This is binding because not only did Cabinet commit to that at the end of last year, but all the parties who make up the Government have committed to being bound by the decision of the electorate."
National leader Simon Bridges said the Cabinet paper about the referendum published online "is quite clear" that the referendum will not be binding.
"It requires a passed law where the public have been able to scrutinise it, and we know exactly what the law is - this doesn't achieve that."
Asked if National will commit to keeping the law if it passes, he said: "I would need to see the law and I'd need to have answers to some basic questions like, what's the tax rate going to be, will gangs be able to legally sell drugs in New Zealand?"
Prime Minister Jacinda Ardern argued that the three parties putting the question into the public "have committed to abide by what the public decide, and the National Party are free to do the same".
"It's up to individual parties to themselves commit to make anything binding, whether it's gone through Parliament or whether it's a draft piece of legislation."
Green MP and spokesperson for drug law reform Chlöe Swarbrick said the party's preferred position was to see legislation passed through Parliament before the referendum so it was 'self-executing' with a majority 'yes' vote.
"But we didn't gain consensus on that step. As it is, a yes vote will be informed by a clear regulatory regime set out in draft legislation that people will know and understand."