Employers that "churn" through subsidised interns under the Turnbull government's plan to get young Australians off the dole queue will be identified and weeded out of the system, officials insist.

Prime Minister Malcolm Turnbull was forced on Friday to defend the $840 million internship program, which will pay businesses $1000 to take on young jobseekers for as little as four weeks at part-time hours.

Unions are warning of a "perverse incentive" for employers to keep turning over interns rather than putting on paid workers and on Friday the nation's peak welfare group, the Australian Council of Social Services, pulled back from its initial full support, saying people should be paid an additional $68 a week to participate.

The estimated 100,000 young Australians who have never done a day's paid work will be offered up to $100 a week on top of their Newstart allowance to work 15 to 25 hours in the "PaTH" program - an acronym for Prepare, Trial, Hire.