Department of Conservation (Doc) staff and contractors have restored the circuit by forcing a new track through the area, which lies between Daleys Flat Hut and Bedford Bridge.
Doc Wakatipu operations manager Geoff Owen said the work, which began last winter, had cost in the ''hundreds of thousands'' of dollars.
It had required rerouting 2.7km of track, rock blasting along 110m of rock bluffs, installing a 12m steel truss bridge and 13m of stairways, and hand-clearing debris in all weather.
The project had been a ''long journey'' for Doc and trampers, but the Rees-Dart's stunning landscapes had made the hard work worthwhile.
The new section is about two hours' walk from the Dart Valley road end, north of Glenorchy, in the Mt Aspiring National Park.
Parts of the track were washed away by river erosion caused by a massive landslip in the valley in January 2014, and the erosion also claimed a temporary bypass track a year later.
The slip also caused a 3km-long lake to form, which Mr Owen said appeared to be ''here to stay''.
Doc had to wait for the lake and downstream riverbank to stabilise before a ''safe and enduring'' new route could be found.
Unstable weather had caused further delays by felling large areas of trees and making new sections of track slip, he said.
The Rees-Dart track is a four-to-five day circuit, regarded by Doc as suitable for experienced trampers. It is usually accessed from either the Rees Valley or Dart Valley road ends.
Before the closure, about 2000 trampers a year used the track.
Mr Owen said the new section of track remained challenging, and Doc recommended trampers prepare for ''arduous'' conditions there.
