The NZ 20 metre contours are available as a free download through Quickmap. This will replace the existing Contour layer under the Topo Group in Quickmap. The Height (Z) coordinate can be exported in DXF format for viewing in 3D packages such as AutoDesk if you have Quickmap version 8.3.7402 (or better) released 7 April 2020. Contours updated from Land Information New Zealand on 02/04/20.
To download the 20 metre contours for free in Quickmap, launch Quickmap, click Tools -> Downloads… then click the Download button below the countours you want to download (North / South island) and follow the install instructions.