On cool nights, I often run the torch over the back garden to see what's out and about. (Cool, rainy nights are best but I'll be waiting a while for that combination.) The light picks out the usual suspects —slaters (woodlice), carabid and tenebrionid beetles, millipedes ... and, if the temperature is sufficiently low, slugs and snails.
Although I haven't found any native molluscs in my garden* there are plenty of introduced species. So far, I've identified seven (three snails, four slugs). There may be a fifth slug but it's elusive. (Gastropods being so good at escaping pursuers, you understand.)
Only two species are braving the summer weather: the common garden snail (Cantareus aspersus) and the yellow slug (Limacus flavus). Both have been introduced from Europe and are abundant in southern Australian gardens. Whereas C. aspersus is familiar to most people, L. flavus is not so well-known.
Adults are relatively easy to identify. They're large yellow to olive slugs, marked with paler spots and blotches. There is an internal shell, which is a thin, slightly curved plate about 1 cm long. (Many slugs have some form of internal shell, although it might be nothing more than a few granules of calcium carbonate.)
This species is strongly synanthropic (associated with humans) and is rarely encountered in undisturbed areas, even in its natural range. As a consequence of its hanging around in gardens and cellars, it has been introduced to many places from USA and Canada to the New Zealand and the Cook Islands. (Not deliberately, presumably, but with plants and building material in the days before proper quarantine procdures.)
In his excellent book on introduced snails and slugs of New Zealand, Gary Barker refers to it as a 'rather secretive animal' that is 'infrequently seen'. It's certainly plentiful (and brazen) in my garden—I counted five on the path in one night—but I haven't seen any since. It's probably a matter of being in the right place at the right time. (Oh, and having a mollusc-friendly garden.) I'm going to keep my eyes peeled for the emergence of the other three or four slug species.
*Not surprising. I live a long way from the nearest natural bushland.
Barker, Gary M. (1999). Naturalised terrestrial Stylommatophora (Mollusca: Gastropoda). Fauna of New Zealand 38. Lincoln, Canterbury, New Zealand: Manaaki Whenua Press.