Any (rough gravel track)
Mangahao Dam is one of the oldest continually operating government built hydro stations in New Zealand making the track up behind the dam an awesome place to explore - there's a heap of history in the area.
5 minutes out of Shannon (a small town just north of Levin), you quickly head deep into the Tararua Ranges. The gravel track follows the old river path through the valley until it starts winding up the hill to an amazing view point. From there you quickly drop down again where you'll come across the first dam.
The main road is suitable in most vehicles - it's a rough gravel road filled with potholes, but causes no major dramas. There are however a few side tracks you can take that are a bit more challenging and where 4WD is probably best.
We spent a day exploring the area and it simply wasn't enough by the time we explored all three dams and the few side tracks we managed to squeeze in.