Department of Conservation (Doc) senior conservation services ranger Richard Kennett said rain on Sunday had undercut 100m of the track north of the Bedford Stream, making it impassable.
As a result, Doc had closed the Dart Valley Track between Daleys Flat Hut and Bedford Steam, about two hours' walk from the Paradise road end. Mr Kennett said some of the track had already collapsed into the river, cutting off access.
''The missing track is alongside high vertical bluffs, which will be dangerous and very arduous to bypass.''
The river erosion was a consequence of a massive landslide in the Dart Valley 12 months ago that had also caused a 3km-long lake to form further up the valley.
Another 200m of track nearby was destroyed by heavy rain a fortnight ago, but Doc rangers were able to form a bypass, he said.
The Rees-Dart track is a four-to-five day circuit in Mt Aspiring National Park that is used by about 2000 overnight trampers a year. Another 3000 people undertake day walks from either the Rees Valley or Dart Valley road ends each year.
Doc Queenstown partnerships ranger Chris Hankin said the erosion was continuing, and it would be technically difficult and expensive to create a new route.
''The bypasses have to be designed with the future in mind.
''There's a lot of difficult country to get around, and we have to be mindful of our track standards and public safety.''
He recommended that people intending to tramp the Rees-Dart circuit this summer make other plans.
Trampers could access the upper Dart Valley from the Rees Valley or the more difficult Matukituki Valley-Cascade Saddle route, but would need to backtrack to return home.
People could also walk from Chinaman's Bluff as far as Bedford Stream before returning.
