National has labelled Marama Davidon's interview with The AM Show "amateurish".
On Monday morning, the Green co-leader was unable to give basic details of proposed welfare reform - the same topic that ended former leader Metiria Turei's political career.
In order to "put the heart back into the welfare system", she wants a 20 percent increase to "some benefits" - but couldn't say how much that policy would cost.
"I know I don't have that on my head right now," she told host Duncan Garner.
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She also couldn't say how much is currently spent on benefits.
"I don't know that cost off the top of my head, but it's not enough."
The Green Party is renewing its calls for welfare reform, an issue which was overshadowed last year by their former co-leader's admission of benefit fraud.
Co-leader James Shaw stood by Ms Turei over that before she resigned. He told Newshub he has "confidence" in Ms Davidson.
But National MP Alfred Ngaro says it was an amateur performance.
"You just jump on public television, you turn around and tell people what you want to do. I just think it's a little careless and the public deserve better than that."
Welfare reform is on the Government's agenda, and an advisory group will make recommendations in February 2019.
Social Development Minister Carmel Sepuloni says decisions haven't yet been made on specific policy options.
As for those details that eluded Ms Davidson, the Green Party's own figures show it would cost approximately $900 million a year to boost core benefits by 20 percent.