The issue is Australia, Canada, the United States and a number of other countries do not have stringent enough licensing requirements to bring them up to the requirements of the Vienna Convention (which provides for a 3 year validity permit). Ratifying the Vienna Convention would mean each Australian State would have to
change its licensing requirements to conform to the requirements of the Vienna Convention. For instance, there would need to be endorsements for specific types of vehicles not otherwise classified in the existing schemes set out by the individual state governments:
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Only 88 countries have ratified the Vienna convention most of them European Union countries which by law must comply with these regulations to stay in compliance with relevant EU legislation.
-RooFlyer88