A former detective is warning a black market will still exist and gangs will still thrive even if cannabis becomes legal after this year's election.
Dave Pizzini, a member of the 'Say Nope to Dope' campaign, believes there will be a surge of negative impacts if cannabis is legalised. Kiwis will get to vote on the issue at September's general election.
Under the proposed legislation, THC - the main psychoactive ingredient of cannabis - can be restricted to a maximum of 15 percent by authorities. The legal age for purchasing it will be 20.
Advocates hope the Bill will eliminate illegal supply of the drug while raising awareness of the health risks of using it.
But Pizzini told The AM Show on Monday the proposed potency of the drug won't be enough for those already using it, and gangs will continue to sell to those under 20.
"It'd be a disaster for New Zealand if it's legalised. For a lot of users, 15 percent doesn't do it to get high, so they'll be accessing the more potent cannabis from the gangs and [the gangs will] thrive.
"I wouldn't like to see it legalised at all. I had 35 years in the police and I spent a lot of time interviewing offenders that I arrested that had cannabis issues, and a lot of them told me they regretted touching the stuff because it ruined their lives."
But another former detective, Tim McKinnel, is fronting the 'Vote Yes' campaign alongside other high-profile Kiwis such as former Prime Minister Helen Clark. McKinnel argues the current system is a failure.
"It discriminates, the law is unevenly applied, and I think it is time for a change," he told The AM Show earlier this month.
"I think cannabis we know, in terms of harm, is less harmful than alcohol and so it is a good place to start."
But Pizzini believes legalisation could put further strain on New Zealand's health system, while also devastating lower socioeconomic areas.
"The cost to our mental health system, which is already overburdened, would be horrendous. It would increase crime because cannabis is a driver of crime.
"Our poor neighbourhoods will have a proliferation of pot shops, just like with the liquor shops in the late 1990s," he told The AM Show, adding he believes legalisation would cause "devastation".
Studies have found in places where cannabis is legalised, usage doesn't increase.
Earlier this month, The Drug Foundation executive director Ross Bell said the legislation would put strong controls on cultivation, supply, and use.
Under the proposed legislation, cannabis can also not be consumed in public, there will be childproof packaging, and sales will be limited to licensed premises. It would also allow people over 20 years old to buy up to 14 grams of dried cannabis or its equivalent per day.
A Newshub-Reid Research poll conducted in February found 39.4 percent were for legalisation, 47.7 against, while 11.6 percent didn't know.
Lobby groups have said the legislation strikes a good balance, but some politicians continue to express skepticism over it. Opposition leader Todd Muller has openly said he doesn't support legalisation and plans to vote against it.
