The parameters for converting from AGD66 easting/northings to GDA eastings/northings are below. This is for Zone 56, for other zones, just change the two +zone=xx values. You need to download or build the PROJ4 cs2cs.exe program to run these PROJ command lines.
cs2cs +proj=utm +zone=56 +south +a=6378160 +rf=298.250000000055 +towgs84=-133.0,-48.0,148.0 +to +proj=utm +zone=56 +south +ellps=GRS80 +towgs84=0,0,0
e.g. 330000 6306000 ==> 330103.70 6306190.14
To convert a lat/lon value from AGD66 to GDA94 use the following PROJ command line:
cs2cs +proj=latlong +a=6378160 +rf=298.250000000055 +towgs84=-133.0,-48.0,148.0 +to +proj=latlong +ellps=GRS80 +towgs84=0,0,0
e.g. 153.0 -33.0 ==> 153d0'3.973"E 32d59'54.244"S
Obviously if you want to convert lat/lon to easting/northing you can mix the two.
The New Zealand Government has an online coordinate converter program you can use here.
The following PROJ command converts from Lat/Lon NZGD49 to UTM NZGD2000:
cs2cs +proj=latlong +a=6378388.0 +rf=297.0 +towgs84=84.0,-22.0,209.0 +to
+proj=tmerc +lat_0=0.0 +lon_0=173.0 +k=0.9996 +x_0=1600000 +y_0=10000000 +a=6378137.0 +rf=298.257222 +towgs84=0.0,0.0,0.0
When you run the command just enter the coordinates in the DOS window. e.g. 175 -36.5 and press enter. The cs2cs program will output the corresponding coordinates (Easting=1779140.10 Northing=5958935.94). The results from this will be a few metres less accurate than the NZ Govt web site because that uses a different technique. But a few metres is usually not all that significant - unless you're a surveyor.
You could easily create a DOS batch file called NZGD49LL_to_NZGD2000.bat that contained this string and then run it whenever you need a coordinate conversion of this type.
The cs2cs program has the following command line usage:
cs2cs source_coord_sys +to target_coord_sys
The strings for the various NZ coordinate systems are:
NZGD49 Lat/Lon:
+proj=latlong +a=6378388.0 +rf=297.0 +towgs84=84.0,-22.0,209.0
NZGD49 Map Grid:
+proj=nzmg +lat_0=-41 +lon_0=173.0 +x_0=2510000 +y_0=6023150 +a=6378388.0 +rf=297.0 +towgs84=84.0,-22.0,209.0
NZGD2000 Lat/Lon:
+proj=latlong +a=6378137.0 +rf=298.257222 +towgs84=0.0,0.0,0.0
NZGD2000 UTM:
+proj=tmerc +lat_0=0.0 +lon_0=173.0 +k=0.9996 +x_0=1600000 +y_0=10000000 +a=6378137.0 +rf=298.257222 +towgs84=0.0,0.0,0.0