It's more pragmatism than policy. Selecting overseas based players in the SANZAR countries creates too many travel (and jet-lag) issues. It's not a factor for European players because all those countries are within one or two time zones of each other. But on the odd occassion when a European player enters the Super-15 (as when Danny Cipriani went to the rebels) then they aren't available for international selection either. Argentina permits overseas players because -- not having a domestic competition of their own -- they don't have any choice. If you want to feel sorry for someone consider the problems of the second tier nations like the Pacific Islands, Canada or Georgia. Professionals from those countries, playing overseas, often have to agree to make themselves unavailable to their national side as a condition of their employment contract.
Unless they had been previously granted permission, both SA and NZ don't pick players playing overseas. Australia is no different.
