As far as I know, a reissue will be required.

And yes, if you don't show up for a reserved flight without cancelling that sector in advance, then your whole itinerary is very likely to cancel.

If you were on a paper ticket you MAY get away with canceling the SYD-MEL leg on the phone, pulling that coupon, then showing up for your next scheduled flight as if nothing had happened - but I'd say the odd's a pretty good they would tell you the ticket had to be reissued at the appropriate cost.

Any variation from the ORDER of your sectors on the ticket is going to require a reissue. Date/schedule changes, are of course at no charge.