Originally Posted by LHR/MEL/Europe FF

You haven't said if you are flying in a premium cabin? If so you will have an express card to assist in clearing SYD immigration and security.

Friends of mine did the same connection a few weeks ago, split party (one in business, the others in economy). They arrived at MEL in time for the much earlier flight, and were accommodated on that flight. They were thankful they were, because the queues at SYD looked tight for economy.

But, as others have said - your connection time is legal and hundreds of passengers must do it every day. So QF must have a pretty good idea of passenger flows and making sure everyone gets their flights.