Jeanette Saxby is an advocate for the legalisation of cannabis and cannabis products in New Zealand.

Deaths from synthetic drugs and the surge in methamphetamine use are almost entirely caused by cannabis being illegal, according to a justice advocate.

"Offenders I work with me tell me it is now easier to get these other drugs than to get cannabis," Roger Brooking, a drug and alcohol counsellor, said.

The claim is backed up by recent figures from the Justice Ministry showing meth is set to overtake cannabis as the biggest drug burden on our court system.

STUFF Drug and alcohol counsellor Roger Brooking says National and Labour have been too paranoid to tackle the issue of legalising recreational cannabis.

Brooking, who recently launched a campaign to cut the prison muster to 7000 in six years, said he had no doubt people were dying because successive governments – including Labour – were so paranoid about decriminalising cannabis.

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"If they decriminalised and then taxed it, that would raise $150 million in revenue, which could pay for a substantial increase in addiction treatment."

ROSS GIBLIN/STUFF Greens police and justice spokeswoman Golriz Ghahraman says decriminalisation coupled with proper regulation of cannabis sale is the way forward.

Decriminalisation would also cut policing and court costs by $400m and help slash prison numbers, which would also help save $1 billion from being spent on a new prison at Waikeria.

Greens police and justice spokeswoman Golriz Ghahraman said drug and justice policy had to be be rational and evidence-based.

Political failure around the issue was starkly highlighted by cannabis' criminalisation and the rise of far more harmful and addictive drugs like methamphetamine.

STUFF A Labour/Greens arrangement promises a referendum by the 2020 election to decide whether the recreational use of cannabis should be legal. (File photo)

"It is of no surprise to me that when a drug like cannabis is criminalised - carrying heavy sanction for users and pushed into the hands of organised criminal gangs - that users are either forced to use more harmful or deadly 'legal' synthetic drugs, or through cannabis use more likely exposed to extremely harmful illegal drugs like methamphetamine," Ghahraman said.

Decriminalisation and proper regulation of sale would mean cannabis could be enjoyed responsibly without users being exposed to criminal sanction, which would also keep it away from children and make health care readily available to those who are affected by addiction or other related health issues.

"Let's remember that those health concerns are less severe than what is commonly associated with alcohol, cannabis is not a drug that causes death from overdose, and is of great benefit to many who use it medicinally," Ghahraman said.

KEVIN STENT/STUFF Police Association president Chris Cahill agrees the country needs to have a conversation about legalising recreational personal cannabis use.

Police Association president Chris Cahill said he did not "automatically accept" Brooking's premise that those who could not get cannabis because it was illegal would turn to other drugs such as meth and synthetics, which were also illegal.

Neither did he accept people were dying because cannabis was illegal. However, the association agreed the country needed to have a conversation about legalising its recreational, personal use.

"But that conversation has to be evidence-based and include all groups of New Zealand society, not just the pro and anti-cannabis lobbies but health, education, welfare and justice sectors also need to be invested in the discussion."

ABIGAIL DOUGHERTY/STUFF National's justice spokesman Mark Mitchell says only a handful of people are jailed for cannabis possession alone, so decriminalising it would have very little impact on the prison population.

A Labour/Greens arrangement promises a referendum by the 2020 election to decide whether the personal use of cannabis should be legal.

National's justice spokesman Mark Mitchell said Brooking's argument failed to acknowledge meth was also illegal, and synthetic drugs had been banned since 2014, unless they pass a strict testing regime to show they are safe.

"It also fails to acknowledge that only a handful of people are in prison for cannabis possession alone and that decriminalising cannabis would therefore have very little impact on the prison population," Mitchell said.