Mrs Bennett said experts will carry out robust clinical assessments to determine whether people are recreational users or have a drug dependency.
Mrs Bennett said estimates at the high end put the cost of enforcing the policy at $10 million.
"We really think the real cost is around $3 million for those that will be known as dependent, once this testing comes in. The savings are estimated to be around $10.5 million,'' she said.
She said the crackdown would involve up to 1300 beneficiaries a year getting treatment for drug dependency.
Mrs Bennett said a Ministry of Health report from 2007/2008 said beneficiaries were three times more likely to be cannabis users.
"That would put them at around 20 per cent of beneficiaries, if you went by that number.
"The UN says that New Zealand and Australia have the highest use of cannabis anywhere in the world,'' she said.
Up to 41 per cent of all jobs that go through Work and Income's books require a drug test.
Mrs Bennett said there was 100 per cent drug testing on anyone working on the Christchurch rebuild.
She said the Ministry of Social Development spent $130 million on addiction programmes.
"We do think we can get help for those that need it,'' she said.
Some people on prescribed medicine will be exempt and people with addiction will be supported with their dependency.