The Dun Mountain Trail starts about 1km along Brook Street, 3 km from Nelson CBD. The trail start is well sign-posted and there are various car parking options in the vicinity.
The cycle trail follows the old Dun Montain Railway corridor, NZ's first railway, and as such is a steady and consistent grade for it's entire length. Follow the trail through the Codgers Mountain Bike Park (a recent storm has required considerable harvesting of trees, so this area is less picturesque that in the past, but don't let that put you off, it's only a short section).
Follow the trail signs and you'll ride up past the Nelson Water Treatment Plant. 100m past the Plant you'll take a right fork and continue on for another 500m until you see a trail on your left with a gate (it's up a short rise).
Go through the gate onto a section of trail through recently harvested pine forest (again less picturesque that in the past!). The next landmark you come to is a small walking bridge, followed by a short steep pinch, then a picnic table on your right. A great place to stop for a rest and look out over Nelson and Tasman Bay.
Continue on and you'll come to a four way cross roads. You go straight ahead here (left is up a steep fire road, right is down a steep fire road - both are the wrong way).
From here the trail continues to climb gradually through mature beech forest eventually reaching Third House at 660m above sea level. Third house is the hut, and there is a fresh water source here for filling up bottles, but nothing else.
Depending on fitness levels this section of the Dun Mountain Trail will take you 1-2.5hrs to complete.