The Department of Conservation says a "bumper breeding season" for ferrets during lockdown has raised concerns.
Hawke's Bay DoC rangers were greeted with high numbers of ferret catches in their trap lines, with 20 per cent more caught in some areas of Hawke's Bay.
Thirty-nine ferrets were caught from July 2018 to June 2019, compared to 48 ferrets caught so far in the 2019/20 season, with a few weeks remaining.
A total of 12 ferrets were removed from trap lines around Boundary Stream - a tract of conservation and farmland protected by intensive management of introduced pests.
DoC ranger Bernie Kelly said the significant rise in numbers of ferret's post-lockdown came as a surprise.
"After a lengthy lockdown, it was good to be back walking the trap lines at Boundary Stream along with my teammates. But the very first trap revealed a rather chunky dead ferret," he said.
"Twelve ferrets might not sound like a lot, but considering the numbers we normally find and the damage these guys can do it's significant.
Kelly added: "Ferret catch numbers usually start peaking in May and June, so being able to get back out there now is very timely."
Boundary Stream is home to range of precious species including kiwi, kākā, pekapeka/bats and five species of wētā.
Ferrets pose a different challenge than stoats and weasels for a range of native species, including kiwi.
Autumn is when young ferrets begin to move into new territories. Unlike many other pest predators, ferrets are able to kill an adult kiwi and it takes only one ferret to kill many birds.
Senior biodiversity ranger Denise Fastier said a driving factor for high ferret numbers could be an abundance of their preferred food source – rabbits.
"The area around Boundary Stream is prone to high rabbit populations, especially in dry conditions and it's been a dry summer," she said.
"Lots of rabbits have been seen in the area this year and prey numbers drive predator numbers. However, when the rabbit population starts to drop, ferrets will look for an alternative to their usual favourite meal.
"That's when you might start to see a higher impact on native species like lizards, wētā and birds – something called 'prey switching', which has happened before at Boundary Stream."
Fastier said it is "crucial" to get on top of the mustelid population as soon possible.
There are currently 594 traps in the Boundary Stream trap network, which targets key predator species in the area including rats, mustelids and feral cats.
Options are being considered for managing the ongoing risks, including additional traps, increasing how often traps are checked and using lures, such as ferret bedding, to attract ferrets or mustelids to the traps.
Anybody interested in getting involved in supporting predator control efforts in Hawke's Bay is asked to contact Hawke's Bay DoC.