Hi Sally!
Even if you haven't done too well in high school, most airlines will be looking at Customer Service skills and experience. Achieving top or average marks don't mean much to airlines in Oz. Its all about your customer service experience as I said.
Or...if you have completed a vocational course such as Hospitality or something related to the industy...this will also look good when you are putting in your application.
Hope this helps
Rollz