ITAR (International Traffic in Arms Regulations) and the EAR (Export Administration Regulations) are export control regulations run by different departments of the US Government. Both of them are designed to help ensure that defense related technology does not get into the wrong hands.
ITAR was enacted in 1976 during the Cold War. ( U.S. Government website.)
"Obummer" widened its application in a deliberate attack on the firearms industry to restrict their business. A whole range of firearms, parts and supplies were added to the list of restricted items despite the fact that they are not "defense technology". This, along with other measures that his administration introduced to generally make life hard for the firearms industry, saw a lot of sellers decide that it was easier and safer ( rather than losing their licence and facing penalties ), to stop exporting than to go through the registration process / costs and to keep track of what is or isn't involved.
Some U.S sellers will still export to Australia, it depends what on you want, wether they are up to date with the regs, and if they can be bothered. ( Also depends on wether a certain Australian importer has an agreement with the O.E.M to restrict individual imports, try getting a Leupold for e.g. )