Traveling for a month in Australia and wanted to rent a car. My best friend will be with me but we are both only 18 years old.
From personal knowledge (Sydney local), most of the car rental agencies require you to be 25 years and older. I think you'll be able to find some which rent under that age but they'll probably have an additional surcharge for younger drivers. In NSW it's actually impossible to get a "full" driver's license by 18 so it might be hard to get a rental, or if you do you'll be paying a bit for it.
tldr; you probably can but it'll be pretty expensive.
Pearse almost 3 years ago