Chief Police Officer Rudi Lammers said any legal use of medical cannabis would require strict provisions for the growth and possession and police conducting roadside drug tests could be faced with uncertainty over drivers found to be under the influence of cannabis.

Assistant Commissioner Lammers told the likely final public hearing of a committee considering the proposal from Greens minister Shane Rattenbury that ambiguity still existed in the plan, including around production, distribution and how patients suffering from terminal and chronic illnesses would identify themselves to police.

He said officers would likely see an increase in reports of marijuana growth and possession in the community, conceding the drug was already widely available for sale to recreational users in Canberra.

"ACT Policing strongly supports federally regulated cultivation [of marijuana] and opposes cultivation by individuals or groups of residential properties or private corporations," Assistant Commissioner Lammers said.

"ACT Policing supports a regulatory framework of persons accessing medical cannabis, with medical cannabis available only on prescription and dispensed through pharmacies."

Any presence of cannabis component THC in the system of a driver on ACT roads would lead to increased risk to the public, Assistant Commissioner Lammers said. He warned no change to drug driving laws should be considered.

"If [cannabis] is produced locally, in homes and businesses, I anticipate a tremendous rise in the amount of responses to complaints against production of cannabis."

Assistant Commissioner Lammers told the hearing no assessment of how much additional funding police would need had been completed.

Concerns have been raised about the plan's provisions requiring potential users to apply to the ACT Chief Health Officer for approval to possess and use cannabis as part of pain relief.

Applicants would fall into three categories, including those with a terminal illness and prognosis of death within a year, chronic or debilitating conditions, or serious illness from cancer, AIDS or HIV, multiple sclerosis, spinal cord injury or epilepsy.

Acting Chief Health Officer Andrew Pengilley said any use of cannabis for pain relief and easing of symptoms such as seizure would require regular supplies to ensure the correct dosages and impact.

Dr Pengilley, who appeared alongside ACT Health director-general Peggy Brown, said medical practitioners could be endorsing products without knowledge of their quality and effects.

He questioned the role of the Chief Health Officer and other health officials, saying the benefits to their involvement wasn't clear. Mr Rattenbury has flagged possible changes to the officers involved in the scheme.

"There will be a significant resource impost in running the scheme we're talking about here, of registering and processing all these applications - in an environment where we have a lot of other public health issues we want to direct resources to," he said.

"This is going to be a large drain on those marginal resources."

The committee's report is due to be provided to the Legislative Assembly by the end of June.