The 'D' coordinates in the New Zealand Geodetic Datum 1949 (NZGD1949) were originally calculated using the New Zealand Geodetic Datum 2000 (NZGD2000) and then transformed into NZGD1949 using a grid transformation.
New Zealand Geodetic Datum 1949 (NZGD1949)
New Zealand Geodetic Datum 2000 (NZGD2000)
For more information see the Grid Transformation page. This transformation accounted for some, but not all, distortions in NZGD1949 leading to potential inconsistencies between NZGD1949 coordinates.
NZGD1949 - NZGD2000 Distortion Grid
Surveyors have known for some time that 'D' coordinates do not align perfectly with non-'D' coordinates and this became more apparent during resurveying for the development of NZGD2000. Most inconsistencies are between marks that belong to different regional networks. This occurred because there was no national network of NZGD1949 marks, instead each regional network was adjusted independently. There were differences between positions with historical and current coordinates, even if they refer to the same physical ground marks.
To maintain accuracy and consistency in cadastral surveys, bearings must be computed using coordinates and heights which have been determined by the same method or that are in the same adjustment network.
Note
NZGD2000 coordinates and New Zealand Vertical Datum 2016 (NZVD2016) have been computed in a single network called the National Geodetic Adjustment.