Thanks for your replies, I'm flying on QF9.
Queff : would you recommend that I seek to change to an alternative flight from BNE, being a UK citizen is the clearance any longer at security.
When you've taken the 1900 flight from BNE how late have you been for the outbound international leg?
I've never missed QF9 but have been amongst the last passengers to board after running from immigration, even though I know the shortest route from Domestic to international, so no wandering around! Not a nice way to start a 20+ hour trip!
I will say that the Smart Gates have sped things up and it's up to you, but if you don't mind possibly hanging around Melbourne airport for an extra hour or so, I would change. Have you got lounge access? If so it will be a much more pleasant "hanging around".
Good luck!