Doc Wakatipu operations manager Geoff Owen said the start of the Dart Track had been severely damaged, and only experienced tramping parties could make it through.
The extent of the damage meant remediation work could take "a number of months".
"This is a reminder of just how volatile the New Zealand environment can be."
Wind and snow storms in the valley, which lies in the Mt Aspiring National Park at the head of Lake Wakatipu, felled large areas of trees and washed away about 100 metres of the Dart Track.
Chinamans Bluff, near the start of the track, was particularly hard hit, with whole sections of forest having "peeled off" and landed on the track below, Mr Owen said.
Another section of the track, between Bedford Bridge and Sandy Bluff, remains impassable as a result of a major landslip in 2014.
The nearby Rees Valley remains open for day walkers, but has also been affected by windfall.
Doc recommends that anyone planning to visit the Rees-Dart area should contact one of its visitor centres to get up-to-date information on track conditions.
