Aestivating snails like these White Italian Snail Theba pisana can be removed by hand ideally before breeding starts with wet weather but will need to be active for baits and sprays to work well. https://australian.museum/learn/animals/molluscs/introduced-snails-in-australia/
Interesting facts on garden snails can be found here: http://www.minibeastwildlife.com.au/resources/garden-snails/
Rain at any time of year may activate snails but breeding of pest species in substantial numbers commences immediately the rains of the rainy season break. In Perth this ‘break’ occurs usually/hopefully during mid-autumn. This is considered to be the first 15 mm or more of precipitation recorded over a three day period (or less) and the damp soil which is needed for egg development can be expected to last right through September.