Tell your curriculum co-ordinator as soon as you get back to school next term.
I think you'll probably end up having to do one of those two exams in the morning session. Your physics exam will probably be moved to the morning session because geography and physics exams are both of the same...
http://www.cssa.com.au/Orders/Timetable.pdf
My school is doing the CSSA exams aswell. My exams are spread out pretty well over the two weeks so I can't complain. :)