Police Association head Greg O'Connor is warning politicians they must get any change in drug laws right.
The Government is agreeing with a major study which found a punitive approach to drug offending is doing more harm than good, and is looking at switching to a health-focused approach.
Mr O'Connor said before they legislate, they must understand the implications.
"If you don't get your policy right, all you will do will be empower your gangs, because you'll lead the supply, increase supply, in the hands of the people who're supplying it now, who are generally the organised criminals."
Greg O'Connor said he's not taking a stance on the pros and cons of loosening the laws around cannabis, but if you increase the demand for it you need to look at who is supplying it.
A former Canterbury farmer who's become the biggest cannabis dealer in Colorado, said the legalisation of the drug for recreational use has returned cash-loads to the community.
Cannabis dealer John Lord told Mike Hosking that in Colorado it's not a gateway drug, and in fact the use of it has become less appealing to youth.
"We've had medical marijuana for many years, and now recreational for two years, and [actually] the statistics are showing a downward curve amongst youth. People don't realise that our average customer is 42 years of age."
He said the taxes from cannabis have already overtaken the state income from alcohol.
"Those taxes have been returned to the community in the form of school construction, drug education programmes, and some really positive things have come of it."
The Greens have supported the decriminalisation of cannabis for the past 10 years, although are refining their policy at the moment.
Health spokesman Kevin Hague said the world's moving so fast on the drug issue that we have to keep up.
Mr Hague said we have to move with the times.
"Countries are going to be adopting a variety of decriminalisation and legalisation models, and New Zealand needs to ensure that we move quickly to monitor what other countries have done to figure out what's going to work and then to make change, otherwise we will be stranded on the wrong side of history."
"I expect that the outcome on cannabis is likely to reflect what's emerging around the world, and that's moving towards a regulated but legalised market, in the same way that countries like Uruguay have done."
Drug researcher Dr Chris Wilkins from Massey University said there's growing international appetite for drug policy reform away from criminal sanctions.
"We would recommend that you separate out medicinal cannabis, which is clearly for medical problems, from a regime where you want to make sure that cannabis is available for some recreational use."
Dr Wilkins said there are 12 different regulatory options that can be looked at.
"So it's anything from reducing penalties, to criminalising small amounts for possession, allowing people to grow their own cannabis plants, and organising cannabis clubs."
He said the important thing about all these different options is they don't involve a commercial profit driven market.