"Statistics NZ and the Treasury have changed their quality assurance and communications processes to ensure this problem doesn't happen again," they said.
As a proportion of all Kiwi children, the revised figures show that children in homes below the poverty line increased from 22 per cent in 2007 to 28 per cent in 2010, and have dropped back only slightly to 27 per cent - not 25 per cent as reported previously.
Children's Commissioner Dr Russell Wills said the figures were still too high.
"With 25 per cent it was already pretty important. At 27 per cent it's still important. Really the message is the same, and it's that child poverty is too common and we need to plan for it," he said.
He said there were signs that child poverty had dropped since the latest figures, which relate to 2012.
"I have been pleased to see that the number of children in benefit-dependent households has fallen. There are more parents in work," he said.
"For most, work is the best way out of poverty, and we clearly still have a long way to go on that."
Poverty revised
Children in homes with under 60% of median income after housing costs
Previously reported
2001: 310,000
2004: 290,000
2007: 240,000
2009: 270,000
2010: 270,000
2011: 270,000
2012: 265,000
Revised
2010: 300,000
2011: 285,000
2012: 285,000
Source: Social Development Ministry