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.

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Work on an alternative route to the glacier is expected to start this week.

Garland said because the track was on an existing erosion site, identifying an alternative route not affected by further erosion would be ''challenging'' .

She said the new track would take several months to construct because the area could not be accessed with a digger and the track would have to be established by hand.

Wanaka i-Site spokeswoman Ling Wei said the 10km return track located 54km west of Wanaka was the third most popular summer walking track for visitors after Roys Peak and Mt Iron.

Even in winter five to 10 groups came into the Wanaka i-Site every day to inquire about the condition of the mostly gravel access road and the weather in Mt Aspiring National Park, she said.

An estimated 20,000 people visit the Rob Roy Glacier a year.

''Not being able to walk the track is definitely going to disappoint some people but you could still visit the glacier by helicopter,'' she said.

It is the second Department of Conservation track to be closed in the last few months.

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The Diamond Lake and Rocky Mountain Tracks were closed on May 31 because of a rockfall and remain closed until further notice.