The duck is a vulnerable flightless relative of the Brown Teal (A. chlorotis) confined to the Auckland Islands. Its closest relative is the endangered Campbell Islands Teal (A. nesiotis). All three were formerly treated as a single species. Subsequently the two flightless forms from the Subantarctic were split as a separate species. All three are genetically distinct and treated as separate species for conservation purposes following a phylogenetic species concept. (Daugherty et al. 1999, Kennedy and Spencer 2000). However H&M4 lump all three under the name "New Zealand Teal" presumably to preserve their former status under the biological species concept and the belief that taxonomy should not be driven by conservation concerns. Canon PowerShot SX50 HS.
References:
Carboneras, C., Christie, D.A. & Kirwan, G.M. (2017). Auckland Teal (Anas aucklandica). In: del Hoyo, J., Elliott, A., Sargatal, J., Christie, D.A. & de Juana, E. (eds.). Handbook of the Birds of the World Alive. Lynx Edicions, Barcelona. (retrieved from http://www.hbw.com/node/52874 on 26 February 2017).
Daugherty, C.H., M. Williams, and J.M. Hay. 1999. Genetic differentiation, taxonomy and conservation of Australasian teals Anas spp. Bird Conservation International 9:29-42.
https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0959270900003324
Kennedy, M. & H.G. Spencer 2000. Phylogeny, biogeography and taxonomy of Australasian teals. Auk 117(1):154-163.
https://sora.unm.edu/sites/default/files/journals/auk/v117n01/p00154-p00163.pdf
Marchant, S. & Higgins, P.J. (coordinating editors) 1990. Handbook of Australian, New Zealand & Antarctic Birds. Volume 1, Ratites to ducks. Melbourne, Oxford University Press.
Shirihai, H. (2007) A Complete Guide to Antarctic Wildlife. The Birds and Marine Mammals of the Antarctic Continent and the Southern Ocean. 2nd edition. A&C Black, London.
Williams, M.J. 2013. Auckland Island teal. In Miskelly, C.M. (ed.) New Zealand Birds Online. www.nzbirdsonline.org.nz.