While BA do contract ground-handling to QF, they do have an office in T1. It will most likely staffed in the period before check-in opens and a short period after the flight departs. Usually, airlines do employ one or two local staff as Airport managers, who then manage the ground handling agents. The BA office is on the third level above check-in, very near to the Prayer Room. Follow the signs to the Prayer Room and you shouldn't have too much difficulty finding the room. (Finding someone there may be more of a challenge. Airlines usually post their office hours on the door, and advise how to make contact out of these hours). Sydney airport has recently installed some way-finding info kiosks, so these should help guide you to the office as well. If I remember, I'll see what info I can find when I'm there Monday.