There were #@#! all direct flights from BNE and MEL to LHR and LAX. Everything had to transit through Sydney either inbound or outbound.
Well Sunfish, I've news for you! Go do some research of Qantas schedules during the 1970s and you will see that flights to/from London and Southern Europe all routed out and in via MEL with SYD being at the start and end of the route. You call that Sydney-centric? As for BNE, they enjoyed a BNE/SIN service that connected in SIN to/from LHR.
As you mentioned Ansett, at what airport were there International B747s based and did they ever go near Melbourne? Like Qantas, Ansett would have, or should have, followed the money.