Forest and Bird fears the Waitākere forest and wetlands are at risk, with concerns a warming climate is partly driving the growth of deer and pig numbers in the area.
It is believed some pigs are being released into highly sensitive coastal wetlands, like Whatipu Wetland.
"Wild pigs are devastating the forests from the ground up by eating young trees and seedlings, and ringbarking mature trees," said Ark in the Park programme manager Sam Lincoln.
"Pigs plough up the ground which, as well as destroying new plant growth, is likely to spread kauri dieback."
But conservationists also worry efforts to focus on pig control could be diverted to another problem plaguing the Waitākere forest and wetlands - deer swimming across from Manukau Heads.
Deer and pigs posed a threat to the region's "largest living carbon sinks", which is home to rare native plants and animals, Lincoln said.
"It's heartbreaking seeing the damage that they can do [to the forest], they're amazing at what they do, they're really good at eating trees.
"With a lot of species, when the youngins have all grown up, females have a smaller range and just kind of do their own thing in the area and the males will be the ones that disperse because they're finding more territory, finding females.
"So if there's not a lot going on where they are, they'll just do what they can to get around ... so [the deer] just swam 2 kilometres across the really fragile wetland."
She said they knew there were deer north of the ranges, and work was ongoing with the council and Department of Conservation, but now the animals were coming from the south too.
"We've got rats, we've got possums ... stoats and ferrets, and we're controlling all of those but deer aren't something we can catch in a trap. We really need to not let them get into the forest into the first place.
"It's hard to completely get rid of them."
And pigs and pests were more likely to breed all-year and sustain their numbers due to warmer seasons, Lincoln said.
But it could be difficult to hold feral pigs at their numbers if illegal releases continue amid lengthened tree fruiting times, council contract hunters warned in a report.
"Nikau are heavily fruiting in the middle of winter, which is unheard of, taraire and miro fruiting for up to 4 months. Spike breeding years and potential liberations [illegal deer and pig releases] can easily tip this balance."
Forest and Bird chief executive Kevin Hague described the situation as being at "crisis level".
Waitākere Ranges must be included in any effort to control the numbers of deer and pigs around the country, he said.
RNZ
