How odd that even Hertz Italy states Australians don't need one:
At the time of rental, the driver must present a valid national driver's licence which has been held for at least 1 year. Italian citizens who hold the new European driver's licence, must also show a valid national identity card at the rental counter as proof of their residential address.
An International Driver's Permit (IDP) is required if your national driver's licence is not in Roman script.
An International Driver's Permit (IDP) is also mandatory by law for non-European Union renters except for customers from the following countries who can rent with their national driver's licence and passport: Argentina, Australia, Bolivia, Brazil, Canada, Colombia, Israel, Mexico, New Zealand, South Africa and United States.