I'm doing bba (which means I've done some of the first & second year units you'd do in comm/economics) + psychology (which involves an awful lot of statistics at times), and gotten through fine on 50% 'general' (2u) maths. YMMV.
Ultimately, whatever your marks and whatever the subject, trick is to get into it: Maths/formulae are tools which can seriously help you. If you approach maths with the "I-hate-it-and-just-want-maths-to-die" approach many (from what I've seen) HSC maths students have, you mightn't go so well. If you're doing 2u at school you may already have realised that..
(..that said, the cycnical 4th year student in me says none of it is particularly useful anyway, but don't listen to him
)
Edit: there aren't any maths requirements, I don't believe, for eco/comm. There may, however, be recommendations: find/read them. They'll give you an indication. There is also the numeracy centre on campus, who can help you out with maths for certain subjects, and usually, if you're having maths troubles, you're not the only one in the class. There are consultation hours with tutors/lecturers, too, where they can go through the work and give you solutions in a more one-on-one session.