This is a good question Geoff, these birds are causing me no end of confusion on Po Toi. Also the adults are moulting their tail feathers.
At the risk of the blind leading the blind, there are two points on your birds suggesting Black Drongo.
Firstly, both show a small white 'loral' spot just below the eye, which I think is diagnostic for Black Drongo when it is present but not all Black Drongos seem to have it.
Secondly, the beaks seem too small and not down-curved enough for Hair-crested. Compare this photo of a bird I think is a Hair-crested taken last week.
Other than that, I am treating all these odd tailed birds as Black Drongo unless they are obviously not.
