There are two ways to get to the top of the Devil's Tail track, the Fringed Hill 2WD road which is a gently climbing 2wd road, or the 4WD road which is shorter but at times very steep.
The Devil's Tail is a steep and technical singletrack which has been used in the NZ Downhill Champs several times. There are a few options on the way down but they all go to the bottom. Not good when wet, and walking sections if you don't have advanced skills.
How to get there:
From Nelson City head about 5 km up Brook St until you get to the bottom Tantragee Saddle (the start of the track)
Better description, track map and several other rides at http://www.kiwivelo.com/mountainbiking.htm