Associate Social Development Minister Chester Borrows said that in 95 per cent of investigations the beneficiaries failed to provide the information they were asked for.
He said ministry investigators were well trained to approach their investigations in a "rigorous, sensitive and professional" manner.
Under the law change, money could be recovered from a beneficiary's partner if they "ought to have known" that their partner was committing fraud.
The New Zealand Law Society expressed concern that this rule was too broad and warned MPs that they were introducing a new standard of liability.
Professor Jeremy Finn, a member of the criminal law committee, said that beneficiaries might speak limited English or have limited intellect, and the threshold should be raised to penalise only people who knowingly or recklessly benefited from fraud.
Women's groups felt the ministry already had strong powers of investigation. The ministry introduced data-sharing with the IRD in January to check beneficiaries' earnings against tax paid. In the first six months after it was introduced, 3000 welfare fraudsters were caught.