Niuafoʻouan has traditionally been classified as closest to ʻUvean and Tokelauan, in an East Uvean–Niuafoʻou branch. However, recent research suggests that it is closest to its neighbour, Tongan, as one of the Tongic languages.
The phonology of Niuafoʻou is similar to that of Tongan, with twelve consonants and five vowel phonemes.
Vowels are more centralized when unstressed. /i/ and /u/ are de-voiced under some conditions.
Niuafoʻou has a very simple syllable structure, (C)V. However, it is apparently transitioning towards allowing consonant clusters, due to the influence of foreign languages and the de-voicing of vowels.