Welfare recipients forced into drug and alcohol treatment programs under a tough new regime will be taking the places of people who want to be there, a Senate inquiry has heard.
There are also fears the welfare crackdown could increase crime.
Medical professionals, addiction specialists and community organisations are urging Senate crossbenchers to reject a Turnbull government plan to drug test welfare recipients.
The drug tests - along with the removal of exemptions for welfare obligations based on drug and alcohol dependence and changes to reasonable excuses - are contained in welfare bills examined on Wednesday.
Each of the measures are expected to funnel more people into drug and alcohol treatment services.
Alison Ritter, from the National Drug and Alcohol Research Centre, said there was already an enormous unmet demand for substance abuse services.
Australia treats about 200,000 people a year for drug and alcohol issues, while another 200,000 to 500,000 people are seeking treatment.
"Treatment services are already full and clinicians are fully occupied," she told senators in Sydney.
"All that means is someone else is going to miss out on a treatment place."
Human Services Minister Alan Tudge on Wednesday rejected criticism of the scheme - describing it as "a trial in every sense of the word" - and argued drug tests were commonplace across society.
The government wants to roll out the drug testing pilot across three trial sites - Mandurah in Western Australia, Logan in Queensland and Canterbury-Bankstown in NSW - affecting about 5000 people.
Anyone who tests positive will be shunted onto cashless welfare cards, while those who fail more than once will be referred to medical professionals for treatment.
The Department of Social Services expects only 120 people across the three sites will be referred on for medical treatment, which could range from counselling sessions to residential rehabilitation.
The department is confident there will be options available for those who need it.
However, it does not know how long existing waiting lists are for services across the three trial sites.
Greens senator Rachel Siewert said it was astounding the department compiled a list of services across the trial sites but did not know about the wait times.
"It is becoming increasingly clear that the government has not done its due diligence when it comes to consultation, evidence and information gathering," she told AAP.
Matt Noffs, from drug rehabilitation service the Ted Noffs Foundation, said trying to force an addict to stop or stripping away their welfare would drive up crime.
"Let me tell you, they will rob your house or my house to get the drugs," he told senators.
Professor Ritter warned the drug tests may also push some people onto more harmful, undetectable substances.
"I think that's a likely unintended consequence and that's obviously a big worry," she said.
The Royal Australasian College of Physicians has described the three measures as "at best ineffective and at worst directly harmful".