The Department of Conservation carried out the 'Northern Ruahine Battle for our Birds operation' on November 13-15.
The date of the drop had been kept quiet as the department held fears staff could be threatened. Police had been notified about a DOC staff member being intimidated by a member of the public in Manawatū early last month after the operation had been announced.
The drop over 30,000 hectares was undertaken to battle an expected surge in predator numbers.
Every 2-6 years beech trees flower and produce massive quantities of seed, in an event known as a 'mast', and sampling by the department in February confirmed a mast would occurring in the area this year.
No sign of rats or possums found in monitoring undertaken after the operation on December 7-8.
Of 160 monitoring sites, just one showed any evidence of a mammal and that was a mouse.
In comparison, the pre-operation rodent monitor, carried out in June, recorded 11 per cent tracked by rats, 3 per cent tracked by mice and 11 per cent interfered with by possums.
DOC Lower North Island operations director Reg Kemper said the results were great.
"We expect to significantly reduce predator numbers following a 1080 operation – that's why we do it. Still, we're happy to know we're achieving what we set out to do," he said.
"It is fantastic news for our taonga species, which are particularly vulnerable at this time of year. Less predators means that moulting whio, young robins leaving the nest, and young kiwi will have a much better chance of surviving the summer," Kemper said.
Further post-operation monitoring will focus on the benefits to native species. Whio populations in two catchments will be surveyed using conservation dogs in late summer. Powelliphanta snail populations will be monitored in March 2018.
The Northern Ruahine operation is one of 34 Battle for our Birds predator control operations taking place across the country this year that together cover about 800,000 ha or 10 per cent of public conservation land.
Battle for our Birds is a nationwide predator control programme that supports DOC's goal of protecting threatened species and making New Zealand predator free by 2050.