Welfare and mental health advocates have slammed the drug-test trial, announced in the budget, arguing it will not help people with addiction and stigmatise those who apply for welfare.

But speaking at St Vincent's Hospital in Sydney, Mr Turnbull said the policy was about reaching out and helping those with drug and alcohol problems.

"If you love somebody who is addicted to drugs, if you love somebody whose life is being destroyed by drugs, don't you want to get them off drugs?" he said on Friday.

"This is a policy that is based on love and a commitment to support Australians."

When asked about the scientific basis of the trial, the Prime Minister replied it was based on the "plain common sense" that it was "rational and compassionate" to ensure people were not addicted to drugs and welfare payments should not be used to buy drugs.

The budget revealed 5000 people claiming Newstart and Youth Allowance payments would be subjected to drug and alcohol testing as part of a trial beginning in January 2018. Those who return a positive drug test will have their welfare payments quarantined (which limits how they spend their welfare). If they fail subsequent tests, they will be referred to a health professional.

Mental Health Australia chief executive Frank Quinlan has cautioned there can be long waiting lists for people seeking treatment for substance abuse and forcing people to get help is unlikely to work.

"Compulsory treatment is not as successful as people would like to think it is," he said.

The Australian Council of Social Service's chief executive Cassandra Goldie says drug testing will "only have negative and nasty consequences," describing it as an "invasive and demeaning".

Lawyers question legal basis for trial

Lawyers have also cautioned the push to introduce drug tests for welfare applicants could be in breach of the Privacy Act and open to legal challenge.

Australian Lawyers Alliance spokesman Greg Barns said Commonwealth bodies such as Centrelink have to comply with the Privacy Act, which regulates how personal information is handled. Mr Barns questioned whether welfare applicants could genuinely consent to handing over sensitive information through a drug and alcohol test, if it was tied to receiving income support.

"You've got a very vulnerable person," he said.

"You're getting an individual who is under duress providing very sensitive information."

Mr Barns said the testing raised serious issues in relation to the Australian Privacy Principles contained in the Privacy Act. These stipulate individuals must consent to the collection of sensitive information and that information is "reasonably necessary for one or more of the entity's functions".

Mr Barns added there were other potential issues around self-incrimination, if Centrelink found people to be illicit drug users and that information was passed on to police. However, it is understood current limits on what Centrelink can do with personal information would prevent this from occurring.

Speaking in Canberra on Thursday, Social Services Minister Christian Porter said the drug-testing trial was on solid legal ground.

"We are absolutely assured that this is completely and utterly lawful," he said.

"We are absolutely assured that it represents no significant departure from the fundamental bedrock of the system, which is the system is about mutual obligation."

The Social Services Minister noted that because the trial would only test new entrants to the welfare system, people would in effect be giving consent to the drug test. "It's open to anyone to not accept the payments . . . and remain outside the system".

The Greens are continuing to seek their own legal advice about the drug-test trial, which leader Richard di Natale has described as a "dangerous proposal".

Constitutional law expert George Williams has already downplayed the option of a High Court challenge, noting there is no right to privacy in the Constitution.

Government to use drugs in wastewater to work out trial sites

The trials will be conducted at three as yet unannounced sites across Australia. Mr Porter said the government would rely on a national program that looks at water waste - and can determine pockets of high drug use - to help choose the test sites.

This will be combined with "internal data" on compliance from the Department of Human Services and Department of Social Services as well as information on drug use from the Australian Institute of Health and Welfare. The CSIRO's data arm, Data61, will be asked to help with the analysis.

"We'll put all of that together and identify a broad group of people and then randomly select inside that broad group inside each of the three trial sites," Mr Porter said.

In light of the Centrelink plan, independent senator Jacqui Lambie has called for drug and alcohol testing to also be introduced at Parliament House.

On Thursday, several senior ministers, including Mr Porter, Treasurer Scott Morrison and Finance Minister Matthias Cormann said they would be happy to be tested.

"Show me the bag, I'll blow in it. Happy to," Mr Morrison told BuzzFeed.

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