The evidence below seems to confirm that this is a frightening possibility.
The weird thing seems to be that very few people seem to know about the existence of this sinister type of weapon. Such a device could be detonated around 100ks into the earths upper atmosphere and from my reading the technology to do this need not be unduly sophisticated. The weapon’s explosion would be noiseless at ground level and unless someone was specifically looking into the atmosphere to see a short lived flash they would never know such a detonation had occurred. There would be no associated building structural damage but otherwise the adverse ramifications from such an hostile act would be catastrophic.
For example every computer within the affected zone would immediately be taken out of commission and these would include those relating to public instrumentality’s such as computer managed power generation and water supply plants. Unless especially protected the computers of banks would be similarly disabled. Furthermore all motor vehicles that have computer managed engines would (unless they were protected) no longer function. Computer managed domestic appliances would fail as well. All affected computers would have to be either replaced or repaired. Buildings would not be structurally damaged.
In other words my summation of these words is that if such an explosion/s took place over any country (including Australia) the targeted country and its citizens would suddenly find themselves being compelled to live in what may have been eighteenth century living conditions. Furthermore such adverse living conditions could persist for many years.
If you are a casual reader I believe that the following two url references provide elementary evidence that supports much of what I have said. I believe I have not exaggerated the contents of this blog. For readers seeking more detailed information about this topic I have copied and pasted extracts from a 2008 US Govt commissioned report regarding this type of potential military threat to the the United States of America. I am not certain if there is a later report than this or not.
This video confirms the existence and danger of this already employed nuclear weapon.
This wikipedia article demonstrates the frightening characteristics of this stealthy weapon.
It seems that the Japanese government is beginning to treat this matter seriously.
Report of the Commission to Assess the Threat to the United States from Electromagnetic Pulse (EMP) Attack
……Electronics are used to control, communicate, compute, store, manage, and implement nearly every aspect of United States (U.S.) civilian systems. When a nuclear explosion occurs at high altitude, the EMP signal it produces will cover the wide geographic region within the line of sight of the detonation.  This broad band, high amplitude EMP, when coupled into sensitive electronics, has the capability to produce widespread and long lasting disruption and damage to the critical infrastructures that underpin the fabric of U.S. society….
 For example, a nuclear explosion at an altitude of 100 kilometers would expose 4 million square kilometers, about 1.5 million square miles, of Earth surface beneath the burst to a range of EMP field intensities.
…. Some critical electrical power infrastructure components are no longer manufactured in the United States, and their acquisition ordinarily requires up to a year of lead time in routine circumstances. Damage to or loss of these components could leave significant parts of the electrical infrastructure out of service for periods measured in months to a year or more…
….Electrical power is necessary to support other critical infrastructures, including supply and distribution of water, food, fuel, communications, transport, financial transactions, emergency services, government services, and all other infrastructures supporting the national economy and welfare. Should significant parts of the electrical power infra-structure be lost for any substantial period of time, the Commission believes that the consequences are likely to be catastrophic, and many people may ultimately die for lack of the basic elements necessary to sustain life in dense urban and suburban communities….
….The Commission has concluded that even a relatively modest-to-small yield weapon of particular characteristics, using design and fabrication information already disseminated through licit and illicit means, can produce a potentially devastating E1 field strength over very large geographical regions….
…..the Commission determined that such weapon devices not only could be readily built and delivered, but also the specifics of these devices have been illicitly trafficked for the past quarter-century. The field strengths of such weapons may be much higher than those used by the Commission for testing threshold failure levels of electrical system components and subsystems…
…..A key issue for the Commission in assessing the impact of such a disruption to the Nation’s electrical system was not only the unprecedented widespread nature of the outage (e.g., the cascading effects from even one or two relatively small weapons exploded in optimum location in space at present would almost certainly shut down an entire interconnected electrical power system, perhaps affecting as much as 70 percent or possibly more of the United States, all in an instant) but more significantly widespread damage may well adversely impact the time to recover and thus have a potentially catastrophic impact….
…..an EMP attack potentially could disrupt or collapse the food infrastructure over a large region encompassing many cities for a protracted period of weeks, months, or even longer. Widespread damage of the infrastructures would impede the ability of undamaged fringe areas to aid in recovery. Therefore, it is highly possible that the recovery time would be very slow and the amount of human suffering great, including loss of life….
….Pulse-current injection and free-field illumination testing on a limited number of refrigerators and freezers indicate that some units will fail from low to moderate EMP levels. This testing indicates that substantial numbers of people would have to survive without benefit of refrigerated foods for an extended period, until repairs or replacement refrigerators and freezers could be obtained. Massive food spoilage at stores and regional warehouses is implied….
….Food processing of vegetables, fruits, and all kind of meats is a highly automated, assembly-line operation, largely driven by electric power. An EMP attack that damages this machinery or blacks out the power grid would stop food processing. The work force in the food processing industry is sized and trained to run a largely automated system. In the event of an attack that stops the machines from running, personnel would not be sufficiently numerous or knowledgeable to process food the old-fashioned way, by hand….
….Electrically driven pumps, valves, filters, and a wide variety of other electrical machinery are indispensable for the purification of water for drinking and industrial purposes and for delivering water to consumers. An EMP attack could degrade or damage these systems, affecting the delivery of water to a very large geographic region. Electrical machinery is also indispensable to the removal and treatment of wastewater. An EMP attack that degraded the processes for removing and treating wastewater could quickly cause public health problems over a wide area….
…..Boiling water for purification would be difficult in the absence of electricity. Even most modern gas stoves require electricity for ignition and cannot be lighted by match. In any event, gas also may not be available to light the stoves (see Chapter 5)…
….A prolonged water shortage may quickly lead to serious consequences. People preoccupied with finding or producing enough drinking water to sustain life would be unavailable to work at normal jobs. Most industrial processes require large quantities of water and would cease if the water infrastructure were to fail….
… Federal, state, and local governments do not have the collective capability, if the water infrastructure fails over a large area, to supply enough water to the civilian population to preserve life…
…Based upon results of the Commission-sponsored analysis, an EMP attack would disrupt or damage a functionally significant fraction of the electronic circuits in the Nation’s civilian telecommunications systems in the region exposed to EMP…
Hurricane Katrina in August 2005 damaged cell phone towers and radio antennas that were crucial to the operation of emergency communications. Protracted blackout of the power grid caused generators supporting emergency communications to exhaust their fuel supplies or fail from overuse. Consequently, government, police, and emergency services were severely impacted in their ability to communicate with the public and with each other… Looting, violence, and other criminal activities were serious problems in the aftermath of Katrina….
….An EMP attack is likely to incapacitate the same nodes—cell phone towers and radio antennas—and overtax generators supporting emergency communications for a protracted period, creating the same conditions that incited lawless behavior in the aftermath of Katrina….