IIRC, the QF back door was only open in the evening before. The whole back section of the lounge past the buffet was closed the last time I was there during the day (mid 2017). Maybe things have changed.
I think they can keep the back door closed as they have a door release in the case of emergencies.
oh yeh. They only man the door (and use it) when there are flights from qf leaving from those gates, and for ba in the evening.