The treasurer, Scott Morrison, has confirmed sewage will be tested to find areas of high drug use to trial drug testing of welfare recipients.

In a video interview with Buzzfeed on Periscope on Thursday, Morrison said that areas of high drug use were “the best place to start” the trials and promised the controversial new measure would only be retained if it “helps people”.

Tuesday’s budget announced a trial of drug testing for 5,000 Newstart and Youth Allowance recipients, with positive tests to be used to push drug users on to cashless welfare cards.

Critics in the welfare sector have said the plan will undermine the needs-based safety net and demonise welfare recipients.

Morrison said the trial was about “helping people” because drug and alcohol abuse can stop people getting a job, meeting mutual obligation requirements for welfare and from “being in a position to make good choices for the rest of your life”.

“We want to do practical things that help people get over that issue and to be able to have a better future,” he said.

The program will start in three trial sites in “targeted areas”, which Morrison said would be areas of high drug use because they were “the best place to start with a program like this”.

Asked if the program would use national wastewater test results to find areas of high drug use, Morrison said the government was “innovative, agile and flexible [and] looks at all sorts of new ways to better target [programs]”.

When Buzzfeed political reporter Alice Workman quipped the government was looking at “new ways and new waste” to target welfare, the treasurer agreed and said, “You’ve got to be smart about [it].” He compared the system to Victoria using water to determine which apartments are left empty to apply a tax on vacant properties.

Testing of sewage is a common method to determine drug use, such as one analysis that found while Australians are big users of illegal stimulants compared with Europeans, although their alcohol intake is relatively low.

Morrison said the drug-testing program was just a trial and compared it to the rollout of the cashless welfare card, which started in particular communities.

He said cashless welfare cards had proved “very successful”, which helped people “better use the welfare made available to them”.

“If [drug testing] doesn’t work, we’ll stop it. And if it does work, and it’s helping people, we’ll keep doing it.”

The cashless welfare card trial has cost $18.9m, or $10,000 per participant for fewer than 2,000 people.

The budget papers said the government will extend income management currently in force in the Northern Territory and 13 other locations until 30 June, 2019.

They also confirm that trials of cashless welfare cards in Ceduna and the East Kimberley will be extended to 30 June 2018 and two further locations will get the cards in September, 2017.