A Maori education expert says Te Kohanga Reo National Trust is right to say pre- school children should not be dividing their time between its kura, and early childhood centres where English is spoken.
The trust has a practice of not taking children part time, as it's found they only become fluent in te reo if they attend five days a week.
It's critical of a Ministry of Education policy that allows tamariki to attend more than one early childhood centre.
Korohere Ngapo, from Waikato University, says there is research to show children grasp two or more languages best, if they first become fluent in the dominant one.
He says it's thought children learn a second language best, if they start when they're seven or eight years old when their first language is well established.
Dr Ngapo says children at kura often pick up English at an early age without any formal teaching, because it's commonly spoken outside school.