Yep, I've lost count of the number of times my bag has taken a different route than me! Worse since the security clamp down esp. LHR.
seasoned, did you check your bags right through? If you did, then Quantas should be the ones to trace it. They are trying to duck responsibility if that's the case. Did you file a PIR on arrival? (Passenger Irregularity Report). If so, you should have got a file number. Then try here:
http://mylostbag.com/www.mylostbag.com/
If you didn't file a PIR, go right back to the airport with your baggage tags and badger someone at Qantas until you do and you get a file number. The problem is that your bags will turn up at your destination, but no-one has your name on file so won't know what to do with them. (In any case, have a peek around the arrivals if you can, they might have already arrived and are sitting forlornly in a corner).
If they were checked as far as HK and you intended to check them in again for the next flight, then you should have got a file reference from BA. In that case check the above website. BA will take responsibility to forward your bags to your destination and advise you when they arrive.
I must say that that is bad of Quantas to shrug you off. It is their responsibility, even though the bag was not on the BA flight. Sounds like someone doesn't know their job (or care).
Good luck & let us know how you get on.
cj