AT, or "The New Zealand Flatworm", as it is known in the UK and Eire, is a particularly nasty pest in those countries as it feeds mainly on earthworms. It slides onto a portion of the earthworm and excretes an anaesthetizing agent and the earthworm is then unable to move. Then come the fluids that digest the worm and the flatworm sucks up the soup, leaving behind the soil particles that were once inside the gut of the earthworm. This species is implicated in the reduction of earthworm numbers within pastures, especially those worms that are deep burrowers. After reducing worm numbers, the lack of their burrows causes the farmland to be come water-logged and thence somewhat acid. The result is an increase in the wetland herbage of coarse grasses and sedges thus reducing palatable stock food by as much as 10%. It is possible that this reduction in earthworm numbers has also led to the reduction in moles that feed on earthworms.
Because this flatworm feeds on earthworms there is a great concern that it would not only reduce productivity of farmland but also decrease the natural biodiversity of the new lands it occupies.
It is not a pest in its native environment of the wetter areas of Canterbury lowlands, foothills and Banks Peninsula. Many European earthworms have been introduced into our pastures and this and other flatworms may be involved in the lack of those European earthworms which are deep burrowers - they were eaten before they could become established. The species is also being spread around NZ and perhaps in some localities it could be regarded as an alien in its own country, and there also reduce the local biodiversity.

There are also Arthurdendyus testaceus (green squares on map) and several undescribed, large species all of which could potentially be "exported" to other countries which would then face further biosecurity risks.
All these species are able to secrete an anaethetizing agent following by digestive fluids and care should be taken when handling them as the fluids can be secreted onto the hand after which it itches.
There are a large number of www sites that deal with the problems of invasive species such as this. It greyish-slate, rarely reddish-grey and has characteristic spots within the pale margin and ventral surface.