I would be VERY careful getting any Assassin snails from anyone in Australia, certain governement agencies do keep track of sale sites looking for any trade in illegal items such as Marimo Moss Balls and I dare say Assassin Snails. These authorities have also shown their ruthlessness upholding the letter of the law, and poisoned entire tanks just to kill one illegal creature.
Remember if Assassin snails are here (which I have heard mention of) then they have been illegally smuggled in and not gone through complete quarrentine measures, nor testing to ensure that if they get released they wont cause havoc in the wild.
As it is even Cherry shrimp and Crystal Shrimp are a grey area of legality of keeping, since the original cherry and crystal shrimp where illegally imported and distributed, now they are so well spread across the country in peoples tanks its all put impossible to locate the original imports and prosecute.