The various negative forms serve different functions; e.g. nuku- non-future, naa potential/involitive, nikara habitual and prohibitive. Naa is not treated as a prefix because the vowel does not get shortened - cf. jaa- PST, which surfaces as [ja-]. The form naa also occurs independently as the negative interjection 'no', and as a tag question marker. Nikara occurs independently as a quasi-verbal predicator meaning [NEG.want]. Nuku is found only preverbally; it cannot be stressed and nothing can intervene between it and the verb. For this reason it is treated as a prefix.
Source: Smith 1974-5: 4970
Source: Smith 1977: 150
Source: Smith 1974-5: 5073
Source: Smith 1974-5: 2235
Source: Smith 1974-5: 5314
Source: Smith 1974-5: 5394
Source: Smith 1974-5: 1372
Source: Smith 1977: 146