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A new lake, formed by a landslide in the South Island's Mt Aspiring National Park, could burst its banks and send millions of cubic metres of water down stream.
The Department of Conservation was due to send staff in a helicopter this afternoon to view the natural dam until the weather intervened.
The dam is made up of soft sediment and rock, said spokesman John Gordon.
Two things could happen. The lake could slowly drain or it could burst its banks and cause what is known as a "catastrophic release".
The lake is about 1km long and was formed on the Young River by a landslide. It is tipped to overflow soon, Mr Gordon said.
"If it was a major incident we would lose a swing bridge and part of a track," Mr Gordon said.
Mr Gordon said the Otago Regional Council has warned farmers to move stock and machinery to high ground.
The community of Makarora has been warned to move livestock and machinery from the flats in case there is a flood.
Otago Regional Council's director of engineering Gavin Palmer said there is a meeting with the Makarora community later this afternoon and bad weather has postponed the helicopter flight.
He said the area is sparsely populated and the majority of houses in the area are on higher ground.
"Our advice has been to stay off the flats and away from the river," Dr Palmer said.
He said just how much water could come down the river is hard to estimate.
"But we believe over-topping is inevitable," Dr Palmer said.
He said the council has looked at intervening with the dam but given the location and scale, the council will let the dam take its own course.
