The small animals are the real problem. Possums were brought from Australia to create a fur trade. There is still a possum fur trade. You can buy all kinds of possum products. I have several pairs of possum and merino wool socks, and very warm they are too. The problem is that there are now about 30 million possums in New Zealand. That would make an awful lot of socks. The possums destroy trees, spread TB and destroy native birds. Their introduction had been a disaster. In Australia they are a protected species, in New Zealand they are a pest. Many trees in Australia have possum defences such as spines, prickles or poisonous leaves and possums in Australia have a lot more parasites, and more predators than in New Zealand.
In hard to reach parts of NZ 1080 poison is dropped by aircraft but the use of 1080 is controversial, not least because some people believe they kill other species as well as possums.
The other solution is trapping, but traps have to be regularly checked and reset after each kill. It is very labour intensive. Now a NZ firm has come up with a new kind of trap. This is gas powered and can kill up to twelve stoats before having to be fitted with a new gas cylinder. Also, its design makes it self clearing. Though the trap costs more to buy the labour costs of supporting it are much less, potentially saving millions of dollars. The traps can also kill rats, another species that kill native birds.
The hope is that the new traps will kill stoats, rats and possums, allowing the number of native birds to increase and protecting trees.
Link to newspaper article.
Link to video.
The Maungatautari Restoration Project provides some indication of what New Zealand could be like if the introduced species could be eliminated.