from Proto-Polynesian *awa 'channel, opening in a reef'

the word was adapted to mean "river" in Māori because the large rivers of New Zealand were more similar to channels than to the small streams (Proto-Polynesian *waitafe) the Māori knew before settling New Zealand

Bruce Biggs (1994), “New Words for a New World”, in A. K. Pawley; M. D. Ross, editors, Austronesian Terminologies: Continuity and Change (Pacific Linguistics Series C; 127), Australian National Universit, page 25