Ooooh, they are tough buggers to get rid of!
I have no idea. Our last place we lived in had a vacant block next door and they just came over from there. We set up traps, but they only get one at a time, of course. Chocolate was good for the traps.
I'd also recommend putting all food in containers, and preferably in the fridge. They used to get at my fruit bowl until I started storing the fruit in the fridge.

ETA: Goodness, Curly! I'd never have thought of that one!!!