[3] Two other taxa which were discovered earlier but which have remained undescribed are likely closely related to this moth: a species which tunnels in Machaerina teretifolia in New Zealand, and another in Lomandra longifolia in Australia.

[3] Prior to its scientific description, Watts and Hoare took to calling the caterpillar of this species Fred the thread, which has been promoted as a common name by some websites.

[9] Samples from areas where the plant once grew in central parts of the region of Northland have not been checked to find evidence of mining to indicate presence of the moth.

[11] H. flexilissima apparently has a two-year life cycle, because the moths emerge from their pupae whilst less mature caterpillar instars are still actively tunnelling.

The larvae mine and pupate inside the living stems of Sporadanthus ferrugineus, a large endemic New Zealand rush-like marsh plant.

[3] Because of its morphological distinctiveness, as well as threats to the habitat of its very local host plant S. ferrugineus, H. flexilissima is considered a species of high conservation status.