Association president Chris Cahill told the committee that the bill was effectively telling police to move from a default presumption to prosecute to one of non-prosecution for all drug users, which would lead to a "dramatic" decline in police charges.
While the association did not have a position for or against decriminalisation, he said it was an issue that should have wider public debate rather than be "slipped in" to a bill that was primarily about synthetic drugs.
After the hearing, Cahill told the Herald that if the Government wanted police to actually use discretion, it should drop the clause in the bill that said "a prosecution should not be brought unless it is required in the public interest".
"To us, there is a subtle distinction [if the clause is not dropped] that police officers would see and would say, 'They're actually directing us not to charge people for [possessing or using] any drugs.'"
Law Society spokesman Chris Macklin told the committee that the wording in the bill would make it easy for a lawyer to argue that a prosecution should never have been brought.
National MP Michael Woodhouse asked Macklin: "Can you conceive of, as a prosecutor, receiving advice from police that a prosecution - for possession only - of any drug would ever be in the public interest under this [bill]?"
Macklin replied: "No, I can't on the spot now conceive of that."
The National Party has said the bill would decriminalise all drug use, but that has been rejected by Prime Minister Jacinda Ardern, Police Minister Stuart Nash and Health Minister David Clark.
Ardern said police could prosecute drug users if it was in the public interest.
"There will certainly, I'm sure, be circumstances where that might be the case."
Nash said the bill aimed to take a health approach, not a criminal approach, but it was not decriminalisation.
"If it is the public interest to prosecute, police have the ability to do so, but by and large, we want police to take a health approach because the evidence shows that this is much more effective in getting people out of the web of addiction."
Macklin joined other submitters including the Drug Foundation in telling the committee about concerns over the amount of drugs that, if found by police, are presumed to be for supply; for many drugs the default threshold is 56 grams.
"The [Supreme] Court said that presumption might be justified in terms of the Bill of Rights if the quantity is set so high that the purpose of supply is a 'near certainty', to reduce or avoid the risk of wrongful convictions," Macklin said.
He noted Ministry of Health advice that 56 grams was "about a month's worth for an average dependent user, or a few days' worth for some heavy users".
David Clark said the committee was the right place to consider whether the amount was too low.
Macklin also added his concern about the potential for unconscious bias in the use of police discretion to unfairly impact Māori, a concern shared by the Drug Foundation, the Drugs Health and Development Project Trust and Māori health organisation Hāpai Te Hauora.
Macklin suggested requiring police to report on how it exercised discretion, including statistics on those charged by gender and ethnicity - though Woodhouse suggested that any discrimination could be mitigated by the likelihood that there may not be any prosecutions at all.
Many submitters agreed that synthetic drugs were often used to escape trauma, and it was important not to criminalise society's most vulnerable, but help them.
Bell told media after the hearing that without a substantial increase in drug treatment services, a health-focused approach to reduce drug-related harm was pointless.
"There's huge expectations on the Wellbeing Budget later this month that the Government will prioritise mental health and addictions, but I'm really nervous they won't meet our expectations."
He wanted annual funding for alcohol and drug treatment services to double to $300 million.
"If we don't see those kinds of numbers, I think the Government will be criticised quite widely, and quite correctly."