Flying by helicopter will be the only way to visit the Rob Roy Glacier for several months after the closure of the track on Monday July 22, because of erosion.
Department of Conservation senior ranger Ruth Garland said the erosion was located 2.5km from the start of the track and was discovered by a ranger last week when he was putting up avalanche warning signs.
She said the Rob Roy Valley was highly unstable and ''the erosion was caused by undercutting of the track from the reactivation of an existing slip''.
Walkers will be able to access the valley to Aspiring hut via the West Matukituki Track but the track to the glacier has been closed from the start of the Rob Roy Track at the suspension bridge over the Matukituki River.
Work on an alternative route to the glacier is expected to start this week.