I cannot say that Melbourne was never a tag from Sydney flight but I started to fly on CX ex Sydney in 1974 (when the route that was cancelled many years previously was restarted) and it was always a direct flight SYD / HKG vv on the B707. Melbourne flights started some years later and I have some vague memory that Brisbane may have been a tag on to the Melbourne HK flight
Adalade started as a once a week tag on to the Melbourne - HK flight and grew over the years to twice daily.