Mrs Bennett said drug testing of beneficiaries would start in July next year.
She said reports the drug testing policy will cost more than it will save were untrue.
"We 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.
Mr Shearer said drug testing was hugely expensive and the money should provide meals in schools instead.
Labour's social welfare spokeswoman Jacinda Ardern said the drug testing policy would not properly tackle the problem.
"The bottom line is we do want to get people into work - and if drug use is an issue, we do want to address this."
She said there was merit to addressing drug use that stands in the way of beneficiaries moving into work but said the policy has some serious question marks over it.