Our leading scientists and conservation experts agree that the eradication of small mammal pests — the rodent-mustelid-possum complex — is the single most important challenge for conservation in New Zealand.

In this section:


The threat of pests was identified at the beginning of the 1900s and through the efforts of scientists and experts from the Department of Conservation (DOC), TBFree (OSPRI) and regional councils, it has become increasingly better understood.

Throughout the 20th century, government agencies – mainly DOC, TBFree, regional councils and Landcare Research – invested many hundreds of millions of dollars in battling mammalian predators. Their expertise is now internationally recognised.

At the same time many thousands of New Zealanders, generally as volunteers, individually or in groups, have invested millions of hours killing rats, stoats and possums all over the country. While there have been many success stories, sadly overall the battle is being lost; predator populations are increasing and many native species are in decline.

Basic facts
  • In 1837 the Australian brush tailed possum was introduced into New Zealand to establish a fur trade.
  • Possums are found all across New Zealand (except for parts of Fiordland) and their major habitat is forests.
  • Its diet consists of leaves, buds and flowers, fruits and seeds, tree ferns, bark, fungi, insects, native birds and eggs, land snails and carrion. They have been known to eat adults of several native bird species, as well as native bats.
  • They compete with native birds, insects, bats and lizards for food.
  • They can be a carrier and transmitter of the disease Bovine Tuberculosis (Tb).
  • Males can live for up to nine years and females 12 years.
Where to learn more about possums


There are no native rats in New Zealand; however, there are three types of rat that came with early sailors: Pacific rat/kiore, Ship/common rat and the Norway/brown rat.

Pacific rat/kiore basic facts
  • Introduced from Polynesia in the 13th century.
  • Its diet consists of fruit, lizards, seeds, leaves, insects, eggs and chicks.
  • Blamed for extinction or reduction of flightless beetles, giant weta, land snails, frogs, skinks, geckos, tuatara and bats.
  • They can live for up to a year.
Ship/common rat basic facts
  • Introduced to New Zealand in the late 1700s.
  • Its diet in the bush consists of fruit, berries and fallen seeds in the colder months and on weta, caterpillars, stick insects, cicadas, snails, beetles and grubs, spiders, native slugs and lizards in spring and summer.
  • Largest impact on our native bird species, as they are good climbers.
  • They can live for up to a year.
Norway/brown rat basic facts
  • Didn’t become established in NZ until 1860.
  • They are great foragers and their diet consists of mice, lizards, chicks, birds and small lizards.
  • Tends to live wherever humans live, particularly in urban areas.
  • They have an impact on animals that live, roost or nest near the ground.
  • They can live for up to two years.
Where to learn more about rats

Mustelids (stoats, ferrets and weasels)

There are three mustelid species in NZ: stoats, weasels and ferrets.

Stoat basic facts
  • Introduced to NZ in 1880 to control rats and hares.
  • They are very versatile hunters and good swimmers.
  • Their diet can consist of rabbits, hares, possums, insects (particularly weta), rodents and birds. Lizards, carrion, birds, eggs, hedgehogs and fish are also known to be preyed upon.
  • Stoats are implicated in the extinction of some of NZ’s native species, including the bush wren and laughing owl.
  • They can live for up to one year.
Where to learn more about stoats

Ferret basic facts
  • Ferrets were introduced into New Zealand in the 1880s.
  • Ferrets eat small animals, including birds, mice and rabbits.
  • NZ has the largest known population of feral ferrets in the world.
  • They can be a carrier of the disease Bovine Tuberculosis (Tb).
  • They have an impact on animals that live, roost or nest near the ground.
  • Ferrets can live for up to 10 years.
Where to learn more about ferrets

Weasel basic facts
  • Its diet consists of native birds, insects, lizards and eggs.
  • Will attack prey much larger than itself, including nesting birds.
  • At 20cm they are the smallest of the mustelids.
  • Have a reputation for being cunning and clever.
  • They have an impact on lizards, invertebrates and bird life.
Where to learn more about weasels