The recent 1080-poison drop on Northland's Russell State Forest has been declared an outstanding success.

Photo: RNZ / Lois Williams

The Department of Conservation (DOC) said results showed rats had been all but wiped out and possum numbers had dropped by 80 percent.

The aerial operation in September covered the Russell Forest north of Whangarei and Cape Brett in the Bay of Islands, and was the first since the mid-1990s.

DOC's Northern region manager Sue Reed-Thomas said the results for Cape Brett/Rakaumangamanga were even better, with no rats or mice detected in test traps and possum numbers slashed by 90 percent.

"These results show outstanding success and prove the effectiveness of aerial 1080 to control pests in a large area over a short period of time," she said.

"These low levels of pests have allowed the spring breeding of native bird life to have a higher chance of survival and the overall health of the forest will start to regenerate."

DOC was committed to working with hapu and communities to keep the pest levels low and bring back the kūkupa (kererū), kiwi and other native species to the ngahere (forest), Ms Reed Thomas said.

Forest and Bird's Northland advocate Dean Baigent-Mercer said the results of the 1080 drop were impressive and meant an early Christmas present for the forest and all the life in it.

"There'll hopefully be some flowering northern rata this year, rather than being munched," he said,

In 2015 local kaumatua had given Mr Baigent-Mercer and a friend permission to put a drone up over the forest and see what was happening.

"We saw the big trees were dying. And when we walked through it, it was all but silent ... we saw maybe one or two tui or kererū where there used to be thousands."

The video footage had gone viral, sparking a three-year campaign to raise community awareness of the forest's plight, and a push for urgent action, in the form of a 1080 drop. That had proved controversial.

"Those kaumatua who supported that, they are heroes, to my mind. They put up with the threats, and abuse from people who objected."

Mr Baigent-Mercer has revealed for the first time that there were attempts to sabotage the September 1080 operation, which was based on remote forestry land in the Puhipuhi hills north of Whangarei.

"One of the most foolish things that was done was someone opened the tap on the fuel tank (for the helicopters) and 2000 litres of aviation fuel was drained onto the ground in the middle of the night," he said.

"I mean ... what craziness. The people who are opposing toxins release much, much more toxic stuff into the earth than 1080 itself. Some pretty ridiculous things happened that don't really show those people in a very good light at all."

Mr Baigent-Mercer said it would probably be necessary to repeat the 1080 drop in a few years, to stop the pest populations rebuilding to their former plague levels.

DOC said it would continue to support the work of Russell hapu and Roopu (pest control groups) to implement a 20-year forest health plan with the long-term goal of forest restoration.

How does DOC measure the success of 1080 drops?

Tracking tunnels are standard DOC practice to measure rat, mice and stoat numbers in the forests.

Tunnels with ink pads and bait are placed in predetermined lines at 50 metre intervals both before and after the 1080 operation.

Staff then analyse the footprints in the tunnels to determine the presence of rodents.

In Russell Forest, DOC staff put out 160 tracking tunnels before and after the aerial 1080 operation. Before, 76 percent of the 160 tunnels had pest interference. After, only one out of 160 tunnels had rat footprints.

Possum presence is measured through wax tags which are placed 20m apart and when possums chew the tags it indicates their presence.

Russell Forest is the largest forest ecosystem in eastern Northland and is home to a number of threatened animals and plant species, including the king fern, forest gecko, kauri snail and longfin eel.

Kūkupa were once abundant but are now scarce, while kākāriki and kaka are probably locally extinct, from sustained attacked from introduced animals - possums, stoats, weasels, rats and feral cats.

(Info supplied by DOC Whangarei)