The NewZealandMapGrid (NZMG) is a projection used for describing locations in NZ in terms of a Northing/Easting (much like UTM?) using the NZGD49 datum. It was designed specifically for NewZealand by Dr W. I. Reilly in 1973 to reduce error over the whole country and as such cannot be used for other parts of the world.
Converting NZMG co-ordinates to a Latitude/Longitude co-ordinates (most common being WGS84) involves using complex polynomial equation. Because NZMG uses the old NZGD49 datum, co-ordinates must be converted to NZGD49 to take into account any distortions before applying the equation. Other common datums used that need conversion might be NZGD2000 or WGS84.