Here's what I have so far:

https://jsfiddle.net/bnhwjL6d/

Instead of showing the shape like it is (correctly mapped on geojson.io)

it shows a circle. Projection wrong? I'm only starting out with D3 maps, so I'm not getting anywhere with this...

{
  "features": [{
  "geometry": {
     "coordinates": [[[172.416088, -43.987709], [172.416585, -43.991154], [172.415202, -43.991154], [172.41816, -43.991837], [172.418697, -43.991745], [172.41817, -43.991028],[172.416088, -43.987709]]],
    "type": "Polygon"
  },
  "properties": {
    "name": "6"
  },
  "type": "Feature"
}],
  "type": "FeatureCollection"
}