Doc scientist Dr Graeme Elliott visited Queenstown recently and released details of the operation in the upper Wakatipu area at a public meeting that drew about 50 people. Dr Elliott told the Otago Daily Times that after a heavy seeding of beech trees in many South Island forests during the past summer, known as a beech mast, rat and stoat numbers would rapidly grow.
When the seed ran out in late winter, threatened bird species were at risk of being decimated or could disappear completely from some areas.
Although the beech mast appeared to be lighter in upper Wakatipu, the operation would go ahead in the area because it was home to one of the South Island's key mohua (yellowhead) populations.
Although the three valleys already had a network of more than 1000 predator traps, they were not sufficient after a beech mast, he said.
After a beech mast in 2011, trapping alone was insufficient to protect some native bird species and mohua numbers fell 75% in the Dart Valley.
More than 90% of mohua disappeared from the Dart and Caples Valleys in 2000, after a beech mast, but in the same area in 2006 and 2009, their numbers remained steady after aerial 1080 operations were carried out.
Other threatened birds in the Upper Wakatipu vulnerable to a predator explosion were whio (blue duck), kaka, kakariki, tuke (rock wren) and kea. The operation would cover up to 24,000ha across the three valleys and would take place over two days, separated by a week, in September or October.
The operation would cost Doc up to an additional $8.2 million nationally, which would come out of its existing budget.
The pellets containing the toxin were cylindrical, about 3cm long, weighed 6g and were dyed green and laced with cinnamon, to deter birds. They were dropped from helicopters at a rate of about 1kg a hectare, or one pellet every 60sq m, he said.
All landowners, businesses and other groups directly affected by the operation had been kept up to date by Doc's Queenstown office.
Doc would also temporarily close affected walking tracks, erect warning signs and advertise in local newspapers.
Wanaka conservation services manager Chris Sydney said recently that in the Mount Aspiring National Park, preliminary data from seed monitoring showed a ''potentially moderate'' amount falling in the West Matukituki area but higher amounts in the Makarora area.
Mr Sydney said the size of the area where poisoning was planned had increased ''quite dramatically'' to include protection for more species, such as the rock wren, which lived at higher altitudes.
