Some undergrad degrees have double majors, such as the Bachelor of Design and Bachelor of Arts - meaning when you graduate your transcript will say you have completed two majors. These double majors you complete in 3 years, as opposed to 4 years (I think correct me if I am wrong) at Monash if you do a double degree. This is because double majors come from the same faculty and there are usually subjects that will cross over, ie. Construction & Architecture double major in Design.
In contrast, in the Bachelor of Science you could theoretically complete the subjects that would count as two majors, but you will only be accredited for one. That being said, taking subjects required for a theoretical additional major would be kinda silly as major subjects (specific in depth subjects taken in your final year) are generally harder and more intense.
It is true that if you minor in something it does not show up on your transcript. Its weird, I don't know why and it sucks. But as someone trying to complete a minor, you bet I will be flaunting that shit in my CV like crazy anyway!! An alternative are the concurrent diplomas, which can require an additional year of study. They are like minors or mini double majors and are offered in Music, Informatics, Languages and Mathematical sciences.
So basically, it will depend on your degree, intended major(s) - not all majors are compatible with another!!