Dr Kāretu, of Tūhoe and Ngāti Kahungunu descent, is one of three Directors of Te Panekiretanga o Te Reo Māori (School of Māori Language Excellence) at Te Wānanga o Aotearoa. He is considered a master of te reo Māori (the Māori language) and is recognised nationally and internationally for his knowledge and work in the language.
In 1992, he received the Queen’s Service Order (QSO) for services to the Māori language. In 1970, he was appointed as the first Professor of Māori Studies at the University of Waikato. In 1987 he became the first Māori Language Commissioner at Te Taura Whiri i te Reo Māori (The Māori Language Commission). He continued in the role until 1999. From 2003, Dr Kāretu was the Head of Te Kōhanga Reo National Trust (Māori language and culture preschools).
He has been involved in numerous Māori performing arts competitions as a tutor, a composer, a judge and an organiser. He was the head of the national Māori performing arts competition for many years. He is also one of the three New Zealand Fellows of the International Centre for Language Revitalisation.