I can give a bit of advice since I do an eco major at usyd. If you meant 'advance maths' as in the course 2u maths, then according to the uni thats the assumed knowledge. I'm not sure if they actually check that since when I enrolled in my econ class, it only came up with a message saying that adv math was a prerequisite and all you had to do was press ok on that message box to enrol, so I'm not sure if they actually check your HSC background to confirm you did it.
In terms of whether the maths is difficult, I would say that it does require some commitment. The introductory econ classes themselves aren't too intensive in math, it's mostly applying linear relationships, calculating areas from graphs when lines intersect, and simple algebraic differentiation and solving of equations. If you do econ as a degree rather than a major you will also need to do introductory econometrics classes - basically introductory statistics and this is where there is a significant amount of mathematical learning. All of it is taught with the assumption of no strong mathematical background, though students that did and excelled in 2u and 3u would already know and be proficient in some of the things learnt.
I've heard that in the following years, econ does get more mathematical. As far as I know, almost every econ degree would require some econometrics courses (you might want to check what UNSW says about this, even if they have no specific pre-req level of maths) and even if it doesn't the study of econ itself is very graphical, equation-based and concept-based so mathematical thinking helps. Though if you want to avoid math you could always take a less quantitative econ major depending on what your uni allows, e.g. environmental economics, economic policy, even political economy if you're inclined to do so.
I'm happy to answer any more questions. Hope this helps.
