Mammal predators pose the biggest threat to native reptiles. Kiore or Pacific rats (Rattus exulans) arrived with Polynesians about 1250–1300 AD and caused the extinction of some lizards on the mainland and on the smaller islands. In the late 1700s and early 1800s, European rats, mustelids (ferrets, stoats and weasels) and cats caused another wave of local extinctions.
The past and current distributions of Duvaucel’s gecko show the impact of predators. Before predators arrived, the geckos occurred on the North and South islands, from Northland to at least as far south as Otago. Today they survive only on a few predator-free islands from Cook Strait northwards.
At least three lizard species have become extinct since people first discovered New Zealand. Eight species have become extinct on the mainland but survive on islands where there are few predators. Those surviving on the mainland are less common than they once were.
Large, nocturnal species are most likely to become extinct, as mammalian predators usually hunt at night. For example, all large species in the genus Oligosoma are nocturnal. Large species are all but restricted to rat-free islands, and small species still occur on the North Island. All were once widespread across the North Island. Even large, day-active lizards are vulnerable to predators and today occur only in small, widely separated populations – just a portion of their original range.
Almost half of New Zealand's reptiles are threatened or endangered. It is now illegal to handle or keep in captivity any native lizard without a permit from the Department of Conservation.
Some, such as the spotted skink (Oligosoma lineoocellatum), are still widespread, although their numbers are probably dwindling. Others are endangered, including the Otago and grand skinks, two of the country’s largest (up to 30 centimetres long). They live in mountain tussocklands in Central Otago, but occupy only one-tenth of their original range. The reason for this is unclear. They may have fallen prey to cats, especially in winter, when cats are hungry and lizards slow. Natural fires and agricultural development are also threats. Research is under way to manage these hazards and help these species recover.
Until recently there have been few attempts to save endangered or rare reptiles from extinction. The eradication of rats from islands such as the Mercury Islands off Coromandel Peninsula has brought new hope for some lizard species.