Aua means: I don’t know.
Aua is interesting to me, because I’m not entirely sure what part of speech it is. Te Aka lists this use as an “interjection” but it’s not covered in the grammars I have as one of the “words that aren’t a particle or a base” so I presume it’s one of the bases. My best guess is that it’s a verb with an idiomatic use that has evolved beyond its normal verbal use. Te Aka lists “aua” as the verb for “to not know”, so I suspect this is a shortened form of “e aua ana au” or similar.
Aua can be used alone as “I don’t know” but other common ways to express this are:
Being introduced with the particle ‘e’ and followed by the particle ‘hoki’ further supports the idea that this is the use of the verb ‘aua’.
Harlow’s grammar includes ‘aua’ in the list of “negative words which are technically verbs but have idiosyncratic uses” bucket, but I think this is a different ‘aua’ which behaves as a shortened form of ‘kaua’. This is either a red herring or there’s a link I don’t understand here, because ‘aua hoki’ is also common in this usage: Aua hoki mātou e kawea kia whakawaia (Lead us not into temptation)