All mammal species in New Zealand except the native ground bats are introduced species, many of them invasive. They wreak havoc on the local ecosystems and have brought quite a few unique bird species near the brink of extinction. I think its absolutely understandable that the New Zealanders want to preserve some of their endemic birds.
The rat trap pictured in the article is interesting. Small CO2 cylinder releases gas into a capture chamber.
From my basic understanding of mammal physiological responses to gases: wouldn't nitrogen gas (as an oxygen displacer) be less stressful than CO2 (as an oxygen displacer)?