Only half of early childhood education providers are doing a good job with children under the age of three, the Education Review Office (ERO) says.
It said 44 percent of the 235 services it reviewed last year were not doing enough to help infants and toddlers talk and explore.
The report said the first few years of life were critical for children's development.
It said the best early childhood services encourage under-threes to try new things, and have teachers that respond well to the children's verbal and non-verbal communication.
But 103 of the services had curriculums that were less responsive or not responsive to the children's needs, and had teachers who could have done more to help their development.
It said most of the poorly performing organisations had the capability to improve.
Educational Institute president Louise Green said the report highlighted the need for more qualified early childhood teachers.
She said the government needed to send a strong message about quality.
"It starts with 100 percent trained and qualified teachers. The government will currently only pay up to 80 percent but some services have as low as 50 percent," she said
"Having trained and qualified teachers in there is really important."
Ms Green said failing to improve the quality of early childhood education would be short-sighted.