iNaturalist follows Clements - not the Checklist of the Birds of New Zealand - for bird taxonomy, and Clements considers the North and South Island Robins to be conspecific.
Clements, J. F., T. S. Schulenberg, M. J. Iliff, D. Roberson, T. A. Fredericks, B. L. Sullivan, and C. L. Wood. 2016. The eBird/Clements checklist of birds of the world: v2016. Downloaded from http://www.birds.cornell.edu/clementschecklist/download/ (Σύνδεσμος)
(Note - due to an error in the taxon change a few months ago that split these taxa [going against Clements], the original iNaturalist taxon concepts for New Zealand Robin and South Island Robin were synonymized, so all New Zealand Robin IDs from the North Island became South Island Robin; if you IDd any robins from the North Island over 5 months ago or so, please check to make sure your IDs are of the right taxon)
Unintended disagreements occur when a parent (B) is
thinned by swapping a child (E) to another part of the
taxonomic tree, resulting in existing IDs of the parent being interpreted
as disagreements with existing IDs of the swapped child.
Identification
ID 2 of taxon E will be an unintended disagreement with ID 1 of taxon B after the taxon swap
If thinning a parent results in more than 10 unintended disagreements, you
should split the parent after swapping the child to replace existing IDs
of the parent (B) with IDs that don't disagree.
(Note - due to an error in the taxon change a few months ago that split these taxa [going against Clements], the original iNaturalist taxon concepts for New Zealand Robin and South Island Robin were synonymized, so all New Zealand Robin IDs from the North Island became South Island Robin; if you IDd any robins from the North Island over 5 months ago or so, please check to make sure your IDs are of the right taxon)