Here is how DOC describes The Coromandel Walkway:
"The coastal walk between Stony and Fletcher Bays crosses farmland and goes through coastal forest with views of the Pinnacles, Great Barrier Island to the north, and Cuvier Island in the east. The track follows an easy gradient with a short steep section in to and out of Poley Bay. It is part of an old bridle path formed by early pioneers to link the two bays."
It's a wonderful, remote ride along the coast generally ridden out and back. As the name suggests it is shared with walkers and there can be lots of them on summer weekends.
If you want to do a circuit there is the option to climb up over the hills on a parallel trail which DOC calls the Coromandel Mountain Bike Track. You'll gain 500 metres (1600 feet), and only ultra fitness freaks will avoid pushing their bikes most the way up - it's steep! The view at the top is awesome and is followed by a steep grassy descent that mainly follows a fence back down meet up with the Walkway. The Walkway is much more fun so most riders just go out and back on that.