ACT leader David Seymour says Greens co-leader Metiria Turei should immediately repay the money she gained from 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 as a solo mother and a law student that she didn't tell Work and Income her flatmates were helping her pay the rent for fear her benefit would be cut.
Ms Turei said on Monday she would pay the money back if she was investigated.
"But what really gets to people is this: she said she would pay it back if she was asked.
"Well here you go Metiria - on behalf of taxpayers I'm asking you to pay it back now."
The Ministry of Social Development isn't saying whether it will investigate Ms Turei.
Deputy Prime Minister Paula Bennett says she's disappointed a senior politician like Ms Turei had not been prepared to play by the rules.
Ms Bennett also received a benefit in the 1990s but says she never deliberately misled officials.
Labour leader Andrew Little says while he doesn't encourage people to break the rules, he thinks Ms Turei was "brave" to speak out.
United Future leader Peter Dunne says he doesn't think there was much point in the confession and it would be vindictive for MSD to prosecute.
Under policies announced at the weekend the Greens want to increase benefits by 20 per cent, remove financial sanctions, increase the minimum wage by $2 to $17.75 an hour and raise the tax rate on income over $150,000 from 33 per cent to 40 per cent.