The South Australian government’s plan to introduce jail sentences for people caught in possession of cannabis faces defeat in the state’s parliament after Labor and upper house crossbenchers signalled their opposition to the bill.

On Monday the SA attorney general, Vickie Chapman, announced a new “war on drugs” in the state with the introduction of a bill to make cannabis a controlled substance on par with ecstasy and heroin.

The bill would mean people caught in possession of marijuana could face a maximum jail term of two years. It would also include tougher penalties for members of outlaw motorcycle gangs. A bikie caught in the supply of a controlled drug would face a $75,000 fine and 15 years jail.

The Guardian reported that harm minimisation experts had warned the bill was based on “nonsense” reasoning and flew in the face of a global move towards decriminalisation of the drug.

But Labor and the crossbench MPs in the state’s upper house have signalled that the bill is unlikely to make it through in its current form.

Labor’s shadow attorney general, Kyam Maher, told the Guardian that while his party would support most of the bill, he did not believe people should face jail terms for possessing marijuana.

“We have serious concerns with that part of the legislation,” Maher said. “For the first time in South Australia under the bill a young person who makes one mistake could end up in jail. The possibility of jail has never been on the statute in this state. We support the other provisions [but] sending a young person to jail for making a silly mistake is not something we’d support.”

The leader of the Greens in South Australia, Mark Parnell, said his party would oppose the bill.

“Further criminalising the possession and use of small quantities of cannabis for personal use is the wrong direction if we are serious about reducing harm in the community,” he said.

“Substance abuse is better dealt with in the health system than the legal system. Evidence shows that waging a war on drugs using criminal sanctions does little to curb harmful behaviour and drives users into the arms of criminal gangs and away from seeking appropriate medical help.”

The government has also raised the idea of limiting the number of times a young person can access a drug diversion program before facing charges. It follows a recommendation from the state’s deputy coroner.

Parnell said the idea was “short-sighted and counter-productive”.

“The attorney general’s comments that those who can’t kick their habit after two attempts are ‘an appalling waste of taxpayers’ resources and goodwill’, shows a frightening callousness and ignorance of the problem of addiction,” he said.

“How many smokers manage to give up tobacco at their first or second attempt to quit? We need to support people with addiction problems rather than throwing them in jail.”

Frank Pangallo from the SA Best party said that while he supported “heavier fines” for cannabis possession, a jail sentence was “excessive”.

“[It seems] harsh to send a person to jail for three offences committed in 10 years,” he said. “They don’t do that for drink drivers. An what about those needing [cannabis] for medicinal purposes? Do you send a dying person to jail?

“Money and resources need to be allocated for rehabilitation programs for not only cannabis but other hard drugs like ice.”

The SA government does not hold a majority in the parliament’s upper house and if all three parties opposed that part of the bill it would not pass.