New Zealand cannabis market already 'enormous', although it's illegal - Helen Clark Foundation

New Zealand's illegal cannabis market is already huge and the harm would be greatly reduced if it were brought out of the shadows, a representative of the Helen Clark Foundation says.

The foundation, led by former Prime Minister Helen Clark, released a report in favour of legalisation of the drug on Tuesday.

Clark said it's time for the drug to be regulated rather than kept as a crime.

"Trying to ban the use of a drug like cannabis just doesn't work. In our own country, it can blight people's lives for a long time if they end up with a conviction, yet the evidence shows that up to 80 percent of us will use it at some point," she told The Project.

Foundation executive director Kathy Errington told Newshub the market is already pretty big and run by people who won't follow the rules.

"We have no way of regulating how strong cannabis is, how old people are who purchase it. But we've got an absolutely enormous illegal market."

She said keeping cannabis in the shadows is preventing education about how to engage with it safely from getting to people.

"It's really hard to educate people how to do something safely if that thing is a crime. 

"You've seen that in the past with anti-HIV campaigns in the 80s, it made it very hard when the thing you needed to talk about was illegal."

The Foundation would also like to see all minor criminal convictions for cannabis expunged if it were to become legal.

Errington said it would be nonsensical to have people walking around with convictions if the drug was being consumed legally.

"If we do pass the referendum and we decide this shouldn't be a crime it doesn't make any sense to keep punishing people for something that we've all agreed is now no longer a crime."

The report also said big business needs to be kept from distribution, to avoid profit motives.