When I was a SAR volunteer a few years ago we would use a 6 digit grid reference using NZTM2000. As in the picture photo below where the lats and longs is converted to NZTM the first and last two numbers are removed from the easting and northing numbers and only the middle 3 are used from each.
Attachment 112474
When the middle 6 numbers are read it is 594 163 so the numbers you have may be 434 775. These numbers will work in a lot of places in NZ depending on what the first 2 digits of the easting and northing is. The last 2 numbers are only needed by the military to drop things on things to make them disappear.