About 36 per cent of first-time mothers, and 13 per cent of mothers who already had older children, were not aware of the Working For Families tax credits.
Even worse, 45 per cent of mothers in the poorest third of the country, who stand to gain the most from the tax credits, were unaware that they exist.
The findings came from the Growing Up in New Zealand study of 6822 families that had babies in Auckland and the Waikato in the past two years.
The study, led by Auckland University researcher Dr Susan Morton, raises questions about the accuracy of Ministry of Social Development claims that 95 per cent to 97 per cent of all eligible families received the tax credits at last count in 2006-07.
The ministry surveyed 5500 families across New Zealand in 2005-06 to find out how many qualified for the tax credits based on their incomes and numbers of children, and how many received them.
It estimated that between 96 per cent and 100 per cent of families that lived solely on benefits, and between 91 per cent and 95 per cent of non-beneficiary families, received their entitlements.
These figures were revised up slightly in 2007.
The Growing Up in NZ study was different because the first results being reported today are based on interviews with parents about three months before their babies were born, so it was possible they found out about the tax credits by the time they had their children.
But Dr Morton said there was no guarantee that the mothers' lead maternity carers (LMCs) or anyone else would have told the families about the tax credits.
"The LMC role is to care for the mother's and child's perinatal health and wellbeing before and up to six weeks after the child's birth - but not to act as an advocate for benefit or other financial supports," she said.
However, a Social Development Ministry spokesman said the ministry's 2005-06 survey found that many families who received the tax credits did not know they were getting them, because they were paid with welfare benefits or accommodation supplements.
Others in the survey would not have been eligible anyway if they earned more than the income limits.
The spokesman said Work and Income ensured all families on welfare received their correct tax credits, and Inland Revenue aimed to reach working families.