Rat numbers have soared to record levels in Auckland's Waitākere Ranges sparking fears for endangered kōkako and other native species.
It comes after a plague of "giant rats" was reported in nearby Titirangi in June.
Forest & Bird's Gillian Wadams said they expected a boom in rats this year due to the predicted "mega-mast", when trees produced large amounts of seeds which rats fed on, but this year was at record levels.
Winter tracking results showed 32 per cent of their tunnels recording rat footprints, up from the last mast season in 2014, and the highest since Ark in the Park was established in 2002.
"We're worried about all our species being hit by this upsurge in rats, and kōkako in particular as they are especially vulnerable," Wadams said.
The tracking levels were well above the 5 per cent that allowed birds to survive and breed, and far higher than the 1 per cent needed for all kōkako chicks to be safe from rats.
They also feared stoat and weasel numbers were much higher than normal too.
"We're heading into bird breeding season, so it's important we get these predators under control now."
The record rat numbers come after nearby Titirangi residents reported a surge in June, with reports of some as big as a cat and roaming in groups of up to 10.
Wadams said they couldn't be certain if the plagues were related, as the Titirangi rats were reportedly gorging on excess chicken feed.
"There was also no evidence the Waitākere rats were larger than usual - just a huge amount of them."
Behind the pest plague is the largest mast seeding event in nearly 50 years.
Mast years typically occur when the average summer temperature was more than a degree higher than the average temperature of the preceding summer.
Before pest predator species were introduced, such mast seasons offered native seed-eating birds a buffet to make up for lean years in between.
But today, with enough predators lurking in our bush to lay waste to some 25 million birds each year, mast years spell even greater danger to those native species still hanging on.
In preparation for this year's mega-mast the Department of Conservation had set aside a $38 million budget for a boost in predator control.
But Forest & Bird said even that did not go far enough, and there needed to be a further boost to predator control efforts. They were also calling on the public to get involved in community trapping groups.
Ark in the Park was a pest control collaboration between Forest & Bird and about 400 volunteers, and landowner Auckland Council, in a 2270-hectare area in the Waitākere Ranges Regional Park.
Forest & Bird spokeswoman Megan Hubscher said climate change was adding to the problem, with masts becoming more frequent and rats able to breed all year during mild winters.
"As well as trapping, New Zealand needs to stay open to using all the tools available for predator control, or we could easily lose many of our most-beloved native animals."
There is no 1080 pest control in the Waitākere Ranges, however Auckland Council has carried out 1080 drops in the Hunua Ranges south of the city where there are similar predator challenges.
In February this year following a 1080 drop the council reported pest numbers had reduced to their "lowest-ever" levels, and led to a doubling of kōkako breeding pairs.