A maximum of 450 welfare recipients are expected to fail the first round of the Coalition government's drug-testing trial, with as few as 90 testing positive a second time, federal government officials say.

The Department of Social Services told the Senate's community affairs committee that between 420 and 450 people might return one positive test, while between 90 and 120 were expected to return a second positive test and then be referred for further assessment by a medical professional.

Officials confirmed these figures were out of the total drug-testing pool of 5000 incoming Newstart and Youth Allowance recipients, who will be randomly selected for the controversial trial. The government has put aside $10 million for a "treatment fund" to help those who test positive.

Doctors and other drug and alcohol experts condemned the trial during a Senate hearing on Wednesday, arguing it was not evidence-based and went against previous expert advice provided to the government.