An imaginary line that connects points of equal height value eg the elevation of the land surface above or below a vertical datum, in this case of LINZ topographic mapping, this is Mean Sea Level.
Data Dictionary for contour: https://docs.topo.linz.govt.nz/data-dictionary/tdd-class-contour.html
Although captured and 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
