The sanction on solo mums who refuse to name the father of their child is making life harder for 16,842 children in low-income households, a government report shows.
Ditching the sanction, which the Government has signalled it will do, would cost $100 million over four years, according to a report from the Ministry of Social Development.
But ministry officials also warn that the costs could be "potentially considerably" higher if levels of compliance fall - a possibility that the National Party has previously warned about.
The advice is contained in a November 10 report, released under the Official Information Act. The report notes that it is unclear whether the policy is working as intended.
The sanction was put in place to encourage the establishment of paternity and child support payments from the father. Failure to name the father sees a $22 weekly penalty on the solo mum, increased to $28 after 13 weeks.