Ms Rebstock, the former chairwoman of the Commerce Commission, was appointed last week to investigate the leak to Labour of a cabinet paper from the Ministry of Foreign Affairs and Trade.
She was also the chair of the Welfare Working Group behind many of the Government's welfare changes.
Ms Bennett yesterday said the board would oversee an "investment approach to welfare."
She said members of the board had experience in the insurance and finance industries which already use investment approaches to reduce future liability.
"The cost of today's total number of beneficiaries is estimated at $45 billion. It makes good economic and social sense to provide targeted support up front to get more people into work sooner," said Ms Bennett.
"This new approach will be embedded at all levels of the welfare system and the board will be responsible for ensuring accountability and overseeing the delivery of reforms that will see fewer people on welfare for long periods," she said.
Critics have slammed the board as being too business-focused, with the appointments having little understanding of the issues faced by people on benefits.
"The new board's mandate to 'oversee the investment approach to welfare' indicates a consolidation of the Government's corporatist approach," Auckland Action Against Poverty spokesperson Sue Bradford told Radio New Zealand this morning.
"The new actuarial approach is a complete break from the philosophy and practice of welfare which has been in place in New Zealand since the 1930s," she said.
"This radical reform is taking place at a time when government economic policies are failing badly, unemployment is rising and the world is facing a very uncertain economic future.
"This is the worst possible moment for the Government to be launching into a model of welfare that is based on ignorance and prejudice rather than well researched and objective policies based on facts and compassion."
However Ms Bennett rejected suggestions it was a "business group".
"It's not business having a greater role, it's people with a range of skills getting involved," Ms Bennett told Radio New Zealand.
"For example, I've got a doctor and a professor on there, one of whom has huge experience in rehabilitation, [and will be] looking at those people, for example, on a sickness benefit and what actually might be helping those people back into work."
She brushed off claims the board lacked experience in the welfare sector.
"I've got fantastic frontline staff, I've got fantastic upper and middle management that are working hands on with policy changes and implementing that frontline," Ms Bennett said.
"What was needed was different thinkers and that is what we have put on the board."
Ms Bennett said the board will report to her, and policy decisions will be up to her. She said she had complete faith Ms Rebstock will stick to the terms of reference and help implement the Government's policy.
"I'm the one, on behalf of the Government, who is implementing policy. I know how I want that policy administered and at the end of the day, I'm at the top of the chain and I will be making decisions as I need to."
