If you had a specific date on the CHC-MEL flight then you should be protected and they should allow you to go one segment over. If you did not then that is one of the risks of open-dating tickets - you are not protected if the flight disappears.

Having said that, the flight seems to exist in my version of the OW Timetable (dep 06:45 MTuThSa). Maybe the problem is that it is operated by JetConnect, but according to the rules as I read them JetConnect is a valid OneWorld / QF affiliate airline for OneWorld Explorer tickets. However, there is a note on some versions of the rules that QF affiliate flights must be booked under QF flight numbers. So as I read it you should still be able to book BA7355 as QF36.