Data Dictionary for tank_poly: http://apps.linz.govt.nz/topo-data-dictionary/index.aspx?page=class-tank_poly
This layer is a component of the Topo50 map series. The Topo50 map series provides topographic mapping for New Zealand and it's offshore dependancies, at 1:50,000. For some small islands the printed map scale is 1:25,000. Although presented at 1:25,000 this layer, for all intents and purposes, forms part of the Topo50 map series.
Further information on Topo50: http://www.linz.govt.nz/topography/topo-maps/topo50
LINZ and our predecessors have been responsible for national topographic mapping in New Zealand and it's offshore dependancies for more than a hundred years.
The raw data for the Pacific Islands of Tokelau, Niue and the Cook Islands was created by photogrammetrists who in the 1970's and 80's mapped the islands from overlapping pairs of aerial photographs. Cartographers then took the data and added symbols and text, and created the colour separations needed to produce printed maps (the NZMS272 series)
In 2013 and 2014 LINZ used 2009 satellite imagery, information from the Ministry of Infrastructure and Planning, Rarotonga, and local knowledge to update the data for the Cook Islands. Printed maps (in hardcopy and digital form) are also available.