Tongoio Scenic Reserve is about 20 minutes outside of Napier in the direction of Gisborne. When driving, you will very suddenly come across the sign, so look carefully on your right side. There is a small parking lot with some nearby picnic tables and shelters, and an explanatory sign about the reserve at the beginning of the path.
For the first 20 minutes of so, you walk along a bubbling stream through a thick forest (although the path is well-defined and fairly wide). Soon after, you veer off to the left and hear the sounds of rushing water. As you come around a corner, you catch your first glimpse of Te Ana Falls, which is the shorter but more forceful of the two waterfalls in the park. The water falls into a small pool that would be a lovely spot for a swim in warm weather. There are also several boulders that provide a good base for a picnic or a place to sit and look at the falls.
From there, you return to the path and head uphill for another 20 minutes to reach an overlook that provides a view of Tongoio Falls across a valley. This waterfall is impressive for its height. You can see the water starting to fall very high on the hill, but the force is soft enough that it mostly just runs down the side of the cliff. Still, though, this is another impressive sight, and the overlook is set amongst beautiful nature.
The two waterfalls are the highlights of the park. However, if you wish to take a longer hike, you can continue walking on the trail all the way to White Pine Bush Scenic Reserve. This takes another hour, so your round-trip walk will be three to four hours. The longer hike passes through a native white pine forest and provides nice scenery, but after the excitement of seeing the waterfalls, I found the end of the long hike to be somewhat anticlimactic. Still, I would recommend this walk on a nice day.