NZGD is an acronym for New Zealand Geodetic Datum. The NZGD is used to accurately describe locations in NewZealand derived from a mathematical model of the Earth. The NZGD is updated from time to time because due to shifting in the tectonic plates that New Zealand spans.
NZGD49 was created in 1949, NZGD2000 was created in 2000 (and will be valid until about 2010). The model used in NZGD2000 is similar enough to WGS84 that coordinates can almost be interchanged between the two with only a small margin of error.
