Wairere Falls is a gorgeous 153m waterfall that plunges in two tiers. I think this could very well be the tallest waterfall on the North Island, though I'm not totally sure about this claim just yet. In addition to the hike to its lookout within the Kaimai-Mamaku Ranges, this waterfall can also be seen from the Old Te Aroha Road, which further indicates the sheer height of it.
For more information on this waterfall such as...
maps
directions
detailed descriptions
refer to A Guide to New Zealand Waterfalls (Story Nature Press).