In Feb 2013, snow and ice was easily avoided. However, even in summer, the south side of North Col can be fully covered in snow / ice. Without appropriate gear, attempting the descent in such conditions would be stupid.
In good conditions, it's a long descent on boulders, scree and snowgrass. The line is occasionally cairned and obvious. From 1200m contour, aim for the gully to the right of the flattening spur. At 1020m contour, cross to the TL of the burn and pick your way down until a slip suggests easier travel on the TR (870m elevation).
From here the going is easy through grass flats - the start of the marked track clearly marked. Continuing through grassy flats, then around a significant tarn and into the bush, the track rises a little then descends to the final flats leading to the Routeburn Flats Hut. There is no bridge crossing the Left Branch but low flow presents no problem.