Mr Bell said the use of warnings was positive but the Government was "irresponsible" for not making legislative changes to reflect how the law was largely being applied in practice.
"You can't just let police make up the law as they go along," Mr Bell said.
"What [John Key is] saying is actually [he's] comfortable with the police not enforcing the law.
"There is a point where politicians do need to be responsible and not accept that it's okay for the police to turn a blind eye."
A spokesperson for Police Minister Judith Collins did not respond directly to Mr Bell's comments, but said police had discretion to warn low-level offenders.
Sensible Sentencing Trust Rotorua spokesman Peter Bentley said he thought the probable reason for such a dramatic drop was because police no longer regarded it as a pursuable offence.
"I do truly think the police are less vigilant about it [cannabis possession]."
Possession of cannabis can result in up to three months' imprisonment and/or a $500 fine.
Rotorua criminal lawyer Rob Vigor-Brown said there were two sides to the issue.
"I think it's really good that young people aren't being convicted for what is a victimless offence because it gives them huge difficulties later in life with job and travel opportunities.
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"On the other hand modern day cannabis is far more potent than it was 20 or 30 years ago and it can lead to psychosis.
"We need to put a considerable amount of effort into educating young people and the dangers of psychosis. It's about education."
Rotorua police area prevention manager Inspector Stu Nightingale said depending on the circumstances a person caught with the drug could be warned instead of charged.
"If the amount of cannabis is very small and they haven't come to our attention previously then they can receive a pre-charge warning."
He said it was explained to them what the consequences would be if they re-offended.
"Police don't look at cannabis offending as being minor and insignificant. Any use of drugs is concerning.
"Cannabis is a gateway drug and can result in worse offences. Police look at each case on its own merits," Mr Nightingale said.
While widely-regarded as not as harmful as other drugs, cannabis-related problems were still a "major part" of a Rotorua alcohol and drug counselling service Te Utuhina Manaakitanga.
"In our experience people have very little understanding of the short or long term effects of cannabis use either physiologically, socially and spiritually," general manager Donna Blair said.
"I would suggest there is poor understanding of the potential harm from all substances whether it be cannabis or alcohol."
Ms Blair said decriminalisation or legalisation would not change the physical, mental or behavioural impacts the drug had on people. She said any reform must be carefully planned before being implemented, and adequately resourced to minimise harm.
Nationally, 17,218 cannabis-related charges were laid in 2009. This figure has dropped each year and was 6492 last year - a drop of 62 per cent.
A number of national bodies, including the Law Commission, Treasury and Institute of Economic Research have highlighted the negative impacts of prohibition and the benefits of cannabis law reform.
The bodies have suggested careful decriminalisation or legalisation would reduce harm for users and result in hundreds of millions of dollars of economic benefits annually.
Mr Bell said the Drug Foundation poll, which was undertaken by Curia Market Research, showed support for a law change across all major political parties.
"Politicians should now be able to proceed with cautious reform without fear of a voter backlash, but any new system needs to protect young people and those communities that already experience the harmful effects of drug use. Any new approach should be carefully designed to improve not worsen drug harm."
- Additional reporting Georgina Harris and Kyra Dawson
Number of people convicted for cannabis possession
Rotorua District and Youth Courts
2009: 153
2010: 137
2011: 109
2012: 101
2013: 75
2014: 55
2015: 52
Whakatane District and Youth Courts
2009: 32
2010: 44
2011: 31
2012: 37
2013: 57
2014: 31
2015: 22
- Source: Ministry of Justice