The 42 Traverse is a great backcountry ride for the adventurous in a fairly remote area. The trail follows old logging tracks for most of the way; there are some steep patches - more down that up, some killer ruts, some fine downhill blats, some river crossings, some fine native forest and some glimpses of the volcones of Tongariro National Park.
You won't want to bike back to the start so you need to arrange a shuttle - easily done at the backpacker accommodation at National Park Village. They also rent bikes. If its a fine day get them to pick you up at the Whakapapa brige where you can freshen up with a swim or you can bike the few kilometers to Owhango where there is a pub.
It is a remote trail and it's not difficult lose it! And it can get really cold in winter. Take care and stuff warm clothing into your day pack!
There's an emerging mountain bike race and a marathon on the trail each year - details at www.t42.co.nz