The Taxpayers' Union says it will invoice Greens' co-leader Metiria Turei for the money she gained by defrauding social welfare when she was a solo mum in the '90s.

Ms Turei made the confession on Sunday when she was highlighting "grinding poverty" in New Zealand and unveiling a policy to increase benefits by 20 per cent.

She told her party's annual conference she had so little money while she was a solo mother and a law student she didn't tell Work and Income her flatmates were helping her pay the rent for fear her benefit would be cut.

Ms Turei hasn't given any details of how much she was receiving at the time, but the Taxpayers' Union says that converted into today's dollars the DPB would be about $366 a week.

"Ms Turei mentioned in the media her fraud lasted three years, that's at least $57,096 she owes taxpayers," the union's executive director, Jordan Williams, said.

"She must pay the money back before she can have any credibility as an MP."

Ms Turei has said she will repay the money if she is investigated.

The Ministry of Social Development won't say whether it will do that.