It depends, at your age I barely studied at all just because the material being taught wasn't really challenging. But it's good to start a habit of studying or at least reviewing what is taught, even if the material isn't hard. I wouldn't say you were smarter than your friends just based on the fact that you don't study as much and get the same grades. If anything, when you are learning harder subjects, those people will be used to studying and have a method down to study effectively. I don't know how schools are taught in Australia, but high school was where I learned my study habits due to the nature of the classes.
College/university is a lot harder in my opinion, but it does vary quite a bit from school to school. College is where study habits are really stressed. Even if you think you are a master of a subject, you still have to put in a shitload of time to review the lessons you thought you would easily get through. If not high school, college is where a lot of people really struggle just because they aren't prepared for the amount of work that is assigned.
