Polygons exist for Auckland=0.13, Christchurch=0.3, Northland=0.1 that have fixed values.
Values outside these regions are interpolated from contour lines which are points spaced at ~1km.
Contours
To fix areas under curves having a constant value after interpolation, lines have been added after initial tracing with average values of surrounding contours.
Added points include all short segments excluding purple (east of north island), the single red point (west coast) and dark green path in the north.
Result
Result includes original contours displayed.