
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.

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.)

______
*Not surprising. I live a long way from the nearest natural bushland.
Read more
Barker, Gary M. (1999). Naturalised terrestrial Stylommatophora (Mollusca: Gastropoda). Fauna of New Zealand 38. Lincoln, Canterbury, New Zealand: Manaaki Whenua Press.