Beneficiaries with children will get an extra $25 a week and families earning less than $27,000 a year an extra $24.50.
Parents who work a certain number of hours can also get an extra $12.50 of in-work tax credits.
Social Development Minister Anne Tolley said the policy was designed to give low-income families a little extra money.
Also taking effect on April 1:
- Paid parental leave is increasing by another two weeks to 18 weeks
- The adult minimum wage lifts by 50c an hour to $15.25
- New Zealand superannuation will increase by 2.7 per cent
- Zero hour contracts will be banned
She said the money should reach more than 380,000 children in low-income families and 190,000 children in benefit-dependent families.
But Green Party social development spokesperson Jan Logie said the amounts were so measly they were just window dressing the real need.
"One of the bits that offends the most about it is that they've taken money from other struggling families through working for families changes to help fund that."
Ms Logie said the poor should not have to pay to marginally improve the lives of the poorest.
Child Poverty Action Group member Susan St John said raising all benefits would have been a better option.
She said the OECD has recommended a catch-up for all adult benefits, not only for those with children.
"Putting a child-related bit into the adult benefit has just muddied the waters."
"Every beneficiary should have had the same percentage increase as a sole parent," she said.
Ms St John said the needs of children would be better addressed by raising the working for families tax credit.
But Ms Tolley said beneficiaries with children and low-income families needed the most help.
She said the increases were the first in more than forty years, and weren't expected to be a one-off.
Mrs Tolley said they were never intended to lift children out of poverty but were a recognition that beneficiaries and low-income households were not reaping the rewards of a growing economy.
"So this was an attempt to lift those people up and give them a bit of extra money."
Labour Party social development spokeswoman Carmel Sepuloni said every dollar counted in some way.
But she was worried the government wasn't addressing children living in poverty.
"We need a real plan from the government. They're yet to present us with one."