The drop in the number of Maori who can speak Te Reo has shocked Statistics New Zealand kaihautu (leader) Luke Crawford.
Census figures issued on Tuesday show only 21%, or 125,000, Maori can hold a conversation in their native tongue - a decrease of 4.8% since the last official count in 2006.
Mr Crawford says the reasons behind the fall are not fully known but there are clues, including decreases in the number of children learning in the Maori mediums of Kohanga Reo and Kura Kaupapa.
Kohanga Reo National Trust Board co-chair Tina Olsen-Ratana says Ministry of Education regulations have contributed to fewer under 15-year old Maori speaking their native language.
Ms Olsen-Ratana says government policies have contributed to fewer tamariki enrolling in kohanga during the past 10 years, which in turn affects enrolments for kura kaupapa.
The ability to speak Te Reo has fallen across all age ranges, except among the over 65s, where it is up 11%.
There has also been a slide in the number of Maori who can speak more than one language, dropping 4.9% to 125,388 people.
Meanwhile, the Maori population has climbed 5.9% to 598,605.
The rise is faster than the increase in the overall population of New Zealand, which has grown by 5.3 % or 214,101 people.
Statistics New Zealand says the biggest surprise in the information is the number of Maori achieving formal qualifications at university.
The proportion of Maori with a bachelor's degree has increased by 56.4% to 36,072.
Hindi is now the fourth most common language in New Zealand, after English, Maori and Samoan.