Hello @choess,
Regarding your assertion above, we maintained the subspecies for the Flora of New Zealand; i.e., treating haurakiense as a subspecies of flaccidum. Pat Brownsey was adamant about this, and looked into this again in detail before writing the eFloraNZ treatment. Have you read his summary here? https://www.nzflora.info/factsheet/taxon/Asplenium-flaccidum-subsp-flaccidum.html
In some places, it is difficult to distinguish the entities. In my experience, while I appreciate that the north-eastern North Island has in parts a particular morphological (and ecological) variant, similar variants occur elsewhere in the range of Asplenium flaccidum s.l. If treated as species, are they flaccidum or haurakiense? Same question for the Kermadec plants mentioned by Pat on the eFloraNZ page.
Until there is some genetic evidence that there are two (or more) distinct evolutionary lineages in sympatry, it seemed more pragmatic to me to keep treating the NE NI variant as a subspecies (i.e., a lineage, albeit only putatively so, with a geographic basis).