The problem with the "I have an ICAO licence" type questions is that it means little. ICAO do not issue licences. Aside from a handful of countries almost every nation on Earth subscibes to the ICAO. That includes The United States and each European nation. Therefore the FAA and JAA are licences that are also issued by ICAO member nations and supra national authorities that in turn are made up from individual ICAO member states.
Each country and supra national authority (in the case of the JAA) will set their own requirements for for the issue, conversion or validation of their own licence when based on a "foreign licence". That requirement will almost certainly depend on the individual circumstances of the licence holder.
It is bit like asking "will I need a visa to visit the USA if I hold a passport issued by a United Nations Member state ?" Who knows, it depends on which member state and your individual circumstances.
Presumably your ATPL is an Australian one and not an ICAO one ?
If you hold an Australian issued licence and you want to work in Europe, then you will ordinarily be required to first obtain a national licence or if applicable a JAA licence from the national authority concerned. The only exception would be if you were flying an Australian registered aircraft or obtained a temporary validation of your own licence from the state in question. The latter is unlikely unless the state was satisfied that a genuine need existed for a temporary period. This sometimes happens when a foreign carrier is leased in to provide aircraft and or crews on a temporary basis.