Green Party co-leader Marama Davidson doesn't know how much her party's welfare policy would cost taxpayers.

The party has renewed its commitment to reforming the welfare system, sticking by the policies introduced by former leader Metiria Turei last year, including a 20 percent boost to benefits.

Asked on The AM Show how much it would cost, she it was a "fair question", but couldn't say.

"I don't have the full cost, but reasonable compared to what we lose when we don't allow people to make their ends meet... Too many people are struggling to make ends meet at the moment, and that's not good enough," she told host Duncan Garner.

"The levels of benefit need to be something where you can actually live, pay the rent and buy healthy kai and pay the power bill and the other bills that come up. They're not, and they haven't been for a long time."

Ms Turei used her personal experiences of being a solo parent on the benefit in the 1990s to promote the Greens' stance, admitting she broke the law to get a few extra dollars. The strategy backfired, with Ms Turei pressured to resign and the party's poll ratings tanking.

Ms Davidson wants benefit rates bumped up by 20 percent - most importantly Sole Parent Support, the modern equivalent of the Domestic Purposes Benefit that Ms Turei cheated on in the 1990s.

"We know that's a real struggle at the moment, that's really tough on children."

She couldn't tell host Duncan Garner how much it would cost, except that it was "absolutely affordable considering we're spending $100 million on America's Cup".