Hike to the Ben Nevis Summit in New Zealand

First thing first, you definitely need a 4WD to reach the car park. The gravel road starts at the beginning of the Pig valley rd. Follow the Wairoa Gorge rd, you'll find a first DOC sign about the Ben Nevis at the crossroad between Wairoa Gorge rd, Bull Creek rd and Old Mill rd. Bull Creek road was closed by a fence when I was there, but you can keep going on Old Mill rd. Don't trust google maps for this part, the road will join the car park, don't worry ;) Just follow the road until the end. You'll understand now why you need a 4WD. The road is narrow, steep and rocky, but not muddy. You'll also cross a stream on the way, pay attention ! The level was very low but I'm pretty sure it can be higher after a couple of rainy days.

There are 2 car parks at the start. One at the lower level, and one just at the start. If you don't have a 4x4, stay at the lower level !

The track is easy to follow, there are not so many crossroads. Follow the orange triangle and you'll be fine. The first part is steep, in a forest (full of bird life), until you reach the ridge (1h). This part is really nice with beautiful views but exposed as you walk close to the edge. It's going gently up, but not so much. 
For the second part, you are back in the forest. You'll find a lot a fallen trees on the track. Avoid them by staying on the right side (1h).
The last part is the final ascent. Completely exposed and steep. Keep going along the ridge until you'll see the metal pyramid at the summit (50min).

Same path for the way back.

Head to the following link if you're looking for the Ben Nevis in Scotland.