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Heavy rain in the Southern Alps is expected to put pressure on a new lake, which experts say could send millions of cubic metres of water downriver if it burst its banks.
The lake - created by a massive landslide in Mount Aspiring National Park - rose half a metre overnight and scientists believe it will inevitably "overtop".
The landslide occurred about two weeks ago and created a lake in the upper Young River Valley some two kilometres in length and 500 metres at its widest point.
The natural dam and the huge body of water behind it have raised safety concerns down river and the public have been advised to keep away from the area.
The lake rose 8 metres over the weekend and a response plan is being developed to ensure local residents are kept informed as the water level rises.
The Department of Conservation's Wanaka area manager Paul Hellebrekers said the water level was 2.5 metres below the banks and heavy rain was forecast in the Southern Alps.
Prior to today's monitoring flight, officials from the three organisations directly involved - Department of Conservation (DOC), Otago Regional Council (ORC) and Queenstown Lakes District Council (QLDC) - met to discuss their plans should there be a sudden breach.
QLDC was working with Makarora's civil defence controller to put a response plan in place. This will ensure all of Makarora Valley's 80 or so residents were well informed about any possible increase in river flows.
Experts did not expect any danger to Makarora residents, but ORC spokesman Gavin Palmer urged people to stay away from the Makarora flats, and to keep away any stock and machinery on high ground.
The area was sparsely populated and the majority of houses in the area were on higher ground.
Meanwhile, aside from the closure of the Young Valley to all visitors, life and business in Makarora continued as usual, a DOC spokesman said.
ORC will today take a video record of the Young River below the natural dam to provide a permanent record for future comparative research after the lake begins to discharge into the lower catchment.
The Institute of Geological and Nuclear Sciences had put a time a lapse camera on the hillside below the dam, which would take a photo every 60 seconds and monitor activity.
- NZPA
