In September 2009, the Topo50 and Topo250 map series replaced the NZMG NZMS260 and 262 maps . These new Topo50 maps are projected in a new coordinate system, New Zealand Transverse Mercator. The NZTM coordinates for a given location will be different from the current NZMG coordinates.
MapToaster Topo/NZ maps for your PC are projected in a NZTM projection. If you have MapToaster Topo/NZ version 4.0 or earlier you should update to the latest version.
MapToaster Mobile/NZ maps for Garmin GPS units are stored in latitude/longitude coordinates. So the GPS maps will not need to be updated.
Your GPS allows you select a range of coordinate systems to display on the screen. You can probably already choose between NZMG, NZTM and lat/long coordinates.