Ministry of Education research shows primary school homework, if handled poorly by teachers and parents, can reverse a student's progress.
The research indicates the best result for students happens when teachers train parents how to support their child's homework.
In that situation, pupils can almost double their progress compared to when parents are not guided properly.
It says the worst thing parents can do is regularly finish their children's homework tasks, because it can reverse their progress by about 20 percent.
The Education Ministry's group manager of curriculum, Mary Chamberlain, says the degree to which homework can damage student achievement was unexpected.