Beneficiaries could ramp up their drug use to avoid sanctions under the latest plan to get them into work, the government has been warned.
Green Party MP Jan Logie says that's the "perverse incentive" under the plan to drug test and penalise them if they don't get jobs because they fail a test, or won't take one.
Those who keep on using drugs, and keep on failing tests, will have their benefit cut - but those who are classified as addicts won't.
Ms Logie says the Ministry of Health has told Social Development Minister Paula Bennett her plan could have absurd effects.
"The ministry advises that the proposal could see recreational drug users pretend they have a more serious problem to avoid penalties, go onto harder drugs that stay in the body for shorter periods to avoid positive tests, or even increase drug use to avoid the recreational category," she said on Friday.
"Heaping a whole new group of people who don't really need treatment onto the health system is absurd."
Ms Logie, and the Labour party, say Ms Bennett should scrap the scheme.
Labour's social development spokeswoman, Jacinda Ardern, says the plan is clearly flawed.
"The minister is simply putting in place an incentive which says `if you come in and fail the drug test and say you're a dependent, we will move you onto the sickness benefit'," she said.
The ministry has also told Ms Bennet the drug testing regime could cost $14 million - about double the amount the government hopes to save by getting more beneficiaries into work.
Ms Bennett says she's working on the details of the policy and knows there's a fine line between recreational drug use and addiction.
She says the alternative is to do nothing and let people continue to draw benefits while they fail to get jobs because they use drugs.