At the moment to be employed by Airservices as a controller you need to be eligible for permanent residency; before being offered work. It's not on the skilled migration list for supported visa's. A few years (maybe 4 years ago) ago a handful of controllers were recruited directly from various locations including South Africa, UK, Sweden, Denmark and possibly some others (without the residency requirements) but that's not happening now. It was essentially stopped due to politics.