This link is pretty close to what I am aftering (Converting NZMG (or NZTM) to latitude / longitude for use with R map library).

Basically I am looking for a formula that can convert NZTM coordinates to lat/long (to be run on C# platform). I need this implemented in a C# code that can take in a pair of NZTM coordinates and return lat/long result. In other words:

public double[] (double nztmLat, double nztmLong){
//Implementation details
//return lat/long in a double array.
}