The Treasury says a coding error in its modelling means it is likely to have overstated how many children would be lifted out of poverty by the Government's Families Package.
The package, announced in December, had been projected to lift 88,000 children out of poverty by 2020-21.
But the Treasury says it has advised the Labour-led Government of a coding error in the modelling.
The error also affected the projected impact on child poverty of the previous National Government's Family Incomes Package, announced in May.
"This is a deeply regrettable mistake and I apologise for it on behalf of the Treasury," Secretary to the Treasury Gabriel Makhlouf said on Wednesday.
"The Treasury holds itself to high standards and I'm disappointed to have not met those standards here."
Mr Makhlouf said the extent of any change in the projections on child poverty was still being determined.
He said the error did not affect the number of people who would be helped by the Families Package, the amount of extra income they would receive, or the fiscal impact of the package.
"Our analysis continues to show that the Families Package will substantially reduce the number of children in low-income households," he said.
Mr Makhlouf said there would be an independent review into the cause of the error and he hoped the revised projections would be available in the second half of February.