Former head of Work and Income Christine Rankin has lashed out at Green co-leader Metiria Turei, calling her "absolutely disgraceful" for committing benefit fraud.
Ms Turei recently admitted she lied while receiving the benefit 20 years ago, and Ms Rankin has called for WINZ to prosecute her.
"I can't believe that someone who wants to be the leader of our country admits to fraud. Benefit fraud is a crime," she told The AM Show.
Ms Turei made the admission at a policy announcement in the weekend, saying she lied about having flatmates out of fear she'd have her benefit cut. She was a law student and raising her daughter at the time.
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Ms Rankin said Ms Turei had done the right thing admitting to what she'd done, but called for her to be prosecuted.
"WINZ need to take action," she said. "They need to prosecute her, which we did to everybody else in her situation 20 years ago - we had a zero tolerance policy. They need to prosecute her and she needs to pay the money back and pay whatever comes from the court."
Ms Rankin said she was surprised there hadn't been an announcement from Work and Income saying they were going to investigate her, in the same way they would any other beneficiary.
"How many other beneficiaries are offered the chance to pay it back? They get prosecuted, like it or not," she said.
"Let's face it, it's not easy on a benefit but no one has the right to commit a crime because times are hard, and they are hard - but it's not provided as a lifestyle, it's a prop-up.
Ms Rankin is herself a former solo mum and beneficiary, and was appointed head of the Ministry of Social Development in 1998. She was a candidate for the Conservative Party in the 2014 election.