Dewes attended Horoera Native Primary School and won a scholarship to Wesley College, where he became dux and head prefect. He went to Ardmore Teachers' College (now part of the University of Auckland) in 1949 and taught at Tikitiki District High School and St. Stephen's Anglican Māori boarding school. From 1962 to 1966, he lectured in the University Extension Department (adult education) of the University of Auckland, and then was appointed as a lecturer in Māori language at the Victoria University of Wellington. At Wellington, he helped to extend courses to allow students to complete a degree major in Māori language. He wrote a master's thesis on the work of composer Henare Waitoa, which was submitted in Māori in 1972. He was awarded an honorary doctorate of literature by Victoria in 2002.
Māori Affairs Minister Pita Sharples said Dewes "was a partisan rather than a diplomat" who "has been an inspiration for language activists from every iwi, and for indigenous peoples around the world."
- "Maori language pioneer dies age 80". New Zealand Herald. 19 August 2010.
- "Historian Koro Dewes dies age 80". Stuff (Fairfax). 18 August 2010.
- "Tohunga fed te reo revival". Stuff (Fairfax). 29 August 2010.
- "People and Places". Te Ao Hou (54): 28. March 1966.
- Ngā waiata haka a Heenare Waitoa o Ngāti Porou (modern dance-poetry by Henare Waitoa of Ngāti Porou) by Koro Dewes
- "Te Runanga". Te Runanga o Ngati Porou. Retrieved 19 August 2010.
- "History of Te Runanga o Ngati Porou". Te Runanga o Ngati Porou. Retrieved 19 August 2010.
- Ihaka, James (19 August 2010). "Maori language pioneer dies at 80". The New Zealand Herald. Retrieved 22 August 2010.