UPDATED 3.05pm: There's a pledge that vulnerable people won't be exploited as a result of bringing private investors into the mental health sector.

New Zealand's first social bond is being introduced, to deliver employment services to people with mental health conditions.

Health Minister Jonathan Coleman says the aim is to help people who want to work, but are struggling to find a job.

"There's no way we're going to have vulnerable people exploited for the purpose of just reaching a finish line in a contract."

Under the scheme, contractors would fund work by selling bonds to investors.

If performance outcomes are met, the Government would pay back the bonds plus a percentage return.

Labour Deputy leader Annette King said no details have been made public about how such payments would be made and the Government hasn't released KPMG reports done in the area.

"Why not put it all out there so we can see what work they've done, what evidence they've got, and why they're choosing mental health?"

Finance minister Bill English told Newstalk ZB's Andrew Dickens an example is a target of getting 100 people with mental disability off the benefit into work and stay at work for at least two years.

"And then you work out what that might cost and if they succeed, the NGO and the private investor get a bit of investor money," English stated.

"And if they don't, well if they don't succeed they don't get extra money."

However, Mental Health Foundation chief executive Judi Clements said no one knew if it would work.

"I think that's something that is quite frightening for some of the mental health service users at the moment who are wondering how it's going to affect them."

The plan's been slammed by the opposition, which says it's offloading the responsibility of a public service.

Clements added only large organisations are likely to attract the right investors.

"The bigger ones might be able to work to get investment in their scheme. I don't think small NGOs will be able to do that."

Despite the foundation's concerns, a business think tank is backing the government's plan.

Executive director of The New Zealand Initiative, Oliver Hartwich, said it was an experiment that would result in a triple win.

"For taxpayers, for patients, for investors and I think it's certainly a scheme worth exploring," he claimed.