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
This layer is a component of the Topo50 map series. The Topo50 map series provides topographic mapping for the New Zealand mainland, Chatham and New Zealand's offshore islands, at 1:50,000 scale.
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 for more than a hundred years.
The first digital data at 1:50,000 was created in the late 80's and early 90's by scanning the 1:50,000 maps that existed at the time (known as the NZMS 260 series, which replaced the imperial NZMS 1 series at 1inch to 1 mile)
The raw data was created by photogrammetrists who from 1974 to 1997 mapped the country from overlapping pairs of aerial photographs. Cartographers then took the data and added symbols and text, and created the colour separations needed to produce the printed maps.
From 1994 to 2006 LINZ used orthophotos to update the map data. Today the map data is updated primarily from aerial and satellite imagery, and data supplied from Department of Conservation, Transit NZ and others.
LINZ releases regular updates of the Topo50 maps and data; for details refer http://www.linz.govt.nz/topography/topo-maps/topo50/update-history
Some features are subject to change more than others. For example in any given map revision, it is likely that road data will undergo more change than, for example, fumeroles. However, all data is examined during a full data revision.