A new way of measuring pupils' performance shows 42% of Maori school children are underachieving in their writing skills.
The first set of results from the Government's new National Standards were released on Friday.
The data also shows 34% of Maori are failing to meet the reading standard.
It's a similar picture for maths, with only 63% reaching the benchmark set by the Ministry of Education.
Education Minister Hekia Parata says it's concerning that a number of Maori are not meeting the standards.
But she says the data will help officials to target students who are falling behind and give them the help they need.