Hey there!
Im in my third year of a BED/BA at Sydney and I'm teaching Hstory and English. What the lecturer meant was that in fulfuling the Arts component of your degree, it is good to do subjects are are quite varied and cover areas in Ancient History, Modern History and Australian History. This benefits you later on when you go for your job interviews in 5th year, and it shows that you have good subject matter knowledge in all areas of history. The Department of Education also looks more favourably on students who have studied as part of their arts component areas of history that are also studied in high schools, eg Ancient Greece.
For example, in my first year I did some Ancient, Modern and Australian History. In second year I did only Modern subjects and this semester I'm doing another Australian history subject. For my fourth year, I'll probably do another Senior Arts unit in Ancient History. Hope this helps, and good luck with the rest of Semester!