The Coalition government has revived controversial legislation that would force 5000 Newstart and youth allowance recipients to undergo drug testing to get their welfare payments.

It's been reported that Prime Minister Scott Morrison is puzzled by the level of opposition to his plan, which has been panned by health experts and welfare groups.

But the Coalition is hoping to pass legislation that would see around 5000 welfare recipients in Logan, Queensland, Canterbury-Bankstown in NSW, and Mandurah in Western Australia tested for marijuana, ecstasy, ice, heroin and cocaine.

UOW law lecturer Dr Ben Mostyn has legal and "common sense" objections to the trial, which would run for two years and be supported by a $10 million dollar treatment fund to provide rehabilitation services for those who test positive.

"The Senate held an inquiry into the drug testing trial bill in 2018 and of the 52 submissions to the inquiry only one was in favour of the trial," Dr Mostyn said.

"From a legal perspective there are issues with such a trial. Forcing people to take drug tests and then forcing them into treatment against their will may contravene human rights laws.

"From a pecuniary perspective, a great deal of research suggests that coerced drug treatment is ineffective. People who don't think they have a drug problem are unlikely to be responsive to abstinence based treatment programs."

Those who fail drug tests under the trial will have to undergo a second test after 25 days and if they test positive again referred to a medic for treatment options.

Welfare recipients who test positive will have 80 per cent of their payment redirected into a cashless debit card that can't be used to buy drugs, alcohol, cigarettes and gambling products.

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"From a common sense perspective, simply because you fail a drug test it doesn't mean you require drug treatment," Dr Mostyn said.

"Cannabis stays in the system for weeks or months so failing a drug test may just mean you smoked a joint a few weeks ago. Most ecstasy and cocaine users only use the drug a few times a year so are unlikely to have a serious drug problem requiring treatment."

Speaking to the ABC's 730, Mr Morrison said the aim of the program was to support drug users by supporting them to quit their habit and it was "worth a try".

But Dr Mostyn said the "expensive program" would violate human rights if it forced people without serious drug problems into ineffective drug treatment.

"If the government wanted to provide holistic and comprehensive mental health services to welfare recipients who asked for such treatment, that would be far more humane and effective," he said.

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