This route can be done as a loop, from the Routeburn car park to the car park by Lake Sylvan, but described here is the shorter out-and-back version over the pass, with a climb up the hill above the saddle.
We got very lucky with the weather, it turned out a sunny, warm day. Before the hike I spent a long time unsure about the snow, how much of it there would be at the top of the pass at this time of year. The doubts only grew as we drove in: the peaks all around were covered in snow. But in the end, either it was a low-snow winter or the pass simply is not high enough to get buried, and it all turned out perfectly passable.
We arrived at the car park at 9:30 in the morning. There was no one else around. Winter is honestly the perfect time here, you get your own private track with no people. It was crisp in the morning, around zero, with frost everywhere. The first thing waiting after the car park is a small swing bridge over the Routeburn river.
You walk along the Routeburn Track for only a moment, and after 800 metres you turn right and start climbing. It is hard to miss: there is a Sugarloaf Pass sign. The route is on Organic Maps, so navigation should not be a problem.
The way up is fairly dull, since for about 2.5 km and 530 metres of gain there are no real views, you walk through dense forest. Though the forest is beautiful in itself: lots of moss and conifers. For most of the way the trail is easy to see, with orange markers on the trees.
The climb is not too hard, almost always uphill, but there are spots where the trail flattens a little and you can catch your breath. At one point you have to cross the Sugarloaf Stream, with no bridge, over fallen trees. In summer you could just take your shoes off and wade through, but in winter we decided against it.
About 2.5 km after the junction with the Routeburn, we came out of the forest. Behind us a panorama started to open up over the Dart River, flowing into Lake Wakatipu.
Above the forest the trail gets very boggy, with streams running straight down it. When we were there, all of it was covered in ice, which was melting in the sun into a slush of grass, mud, ice, and water. By this point the climb keeps getting gentler: the last 800 metres with about 100 metres of gain.
The top of the saddle is fairly hard to pin down, the pass stays level for several hundred metres. We went roughly to the point where the trail slowly began to drop, and a view of the Rockburn valley opened up in front of us. All the surrounding peaks were covered in snow, which made the valley look even grander.
From here you can carry on around the loop and descend to the Rockburn stream and Lake Sylvan. But at the time of our visit the bridge to Sylvan was closed, and it is another 11 kilometres from the pass to the campsite (and then 4 km back to the Routeburn car park). Instead we decided to climb the hill to the right of the saddle. Another 100-150 metres of gain and about a kilometre with no trail.
There is no trail at all there, but there are also no shrubs, dense trees, or streams like on the way to the pass. Scrambling up is not too hard. As you climb, the view over the Rockburn valley shifts, new angles appear, and the mountain peaks on the left come into better view.
When we got to the top, a 360-degree panorama opened up. Right in front of us Mount Earnslaw, to the left the Rockburn valley, to the right Lake Wakatipu and the Dart River, and behind us the mountains above the Routeburn Track.
The unhurried climb, with a stop at the pass, took about three hours. We spent another half hour or so at the top.
It was time to head back. We dropped down to the pass and started descending. The ice had melted even more in the sun, and by the time we reached the forest our feet were soaked through. The descent through the forest was uneventful, following the markers back the other way, and by 3:30 we were back at the car park. Six hours all up for the whole route.
Sugarloaf Pass is not as epic a hike as Mueller Hut, Gertrude Saddle, or the Tongariro Crossing. But for a moderate-difficulty track it gives unusual and beautiful views. On top of that there are almost never any people, and there is a sense of wild, untouched nature. I will definitely have to come back in summer.