EPSG guidance note #7-2, http://www.epsg.org
EPSG initially gave this method the name "Geographic3D to GravityRelatedHeight (NZgeoid2009)". As the same file format was retained for the 2016 geoid, date removed from the method name.
This transformation involves the application of a geoid-ellipsoid separation value interpolated from a quasigeoid model. The model provides separation values at the nodes on a regular grid of latitude and longitude intersection points. The geodetic latitude and longitude used to interpolate within the grid are not affected by this transformation. The grid is referenced to a specific geographic CRS (the horizontal component of the source CRS) and interpolation must be made in the latitude and longitude of this system. Calculation of the separation is achieved through a bi-linear interpolation of the grid, using the latitude and longitude of the point. This step provides the geoid-ellipsoid separation (N) above the ellipsoid of the source Geographic 3D CRS. Then: H = h - N where h = the height above the ellipsoid in the source geographic 3D CRS and H = the gravity-related height in the target vertical CRS.