Bill Stefaniak, who was a Liberal member of the Assembly, said he believed the bill would encourage young people to use drugs and would result in more deaths.

Mr Stefaniak was one of five parties to appear at an emotion-charged public hearing into an inquiry into Drugs of Dependence amendment bill.

Under the bill, put forward by Labor backbencher Michael Pettersson, Canberrans would not face criminal sanctions for possessing small amounts of certain drugs, such as heroin, ice and cocaine.

The first public hearing into the bill on Thursday heard from parents with children who have been affected by drug use. Mr Stefaniak's son Jozef, 24, was killed in a car accident in 2018, and the driver was drug-affected at the time.

Long-time drug reform campaigner Marion McConnell also appeared before the hearing. She said she "wholeheartedly" supported the changes.

"I have great hope and optimism that the ACT assembly through this inquiry, this committee and these hearings will deliver the long awaited reforms that are so desperately needed," Mrs McConnell said.

"I believe criminal sanctions for personal drug use causes more harm than it does good."

Mrs McConnell's 24-year-old son died of a heroin overdose in 1992. She and her late husband Brian started the Families and Friends for Drug Law Reform in 1995, which advocates for a harm minimisation approach to drug laws.

"I really think that criminalising people who use small amounts of drugs, it's not helpful," she said.

Peter Taylor's son also died of a heroin overdose and he also spoke in favour of the bill.

"This is all wrong, I shouldn't be here. It is wrong I'm here because my son died of a heroin overdose," he said.

"The proposed bill on decriminalisation seems to be a momentous change in our thinking. I see it as a first step.

"Since I have been thinking about drug law reform I've hoped that the ACT would show leadership in this area."

But Mr Stefaniak, who was also the ACT's opposition leader from 2006 to 2007, urged the committee members against the bill.

"I think this is an important bill. I don't think it's necessary," he said.

"I think it will lead to a lot of burglaries, a lot more sad parents, a number of people being killed who would not normally be killed."

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Ross and Mary Bingham also spoke out against the bill as parents of a son who has a drug addiction.

"If [the bill] was in place, it makes it easy," Mrs Bingham said.

"A small amount of ice ... there is no such thing as a small amount of ice. Once you take it you're hooked."

The proposed changes would set a possession limit of two grams for cocaine, ice and heroin and 0.5 grams for ecstasy. Those caught with drugs within the possession limits would face a fine of $100 as opposed to going through the criminal justice system.

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