The short answers is yes - you can use any GPS anywhere in the world, but.....IF you buy a US designated GPS it will contain basemaps for thew country from which you purchased. This is not so much a problem for units that use other mapping files over the top basemaps, but for basic, no mapping units that rely on basemaps, you will find there is either no data for Australia or what data there is will be very minimal compared to what is loaded onto an Australian designated unit. You cannot update or modify basemaps either as they are written to the EPROM or brains of the unit.
Cheers,
Brett