"Is there a policy on this? "

The normal policy, I believe, is that there is no pre-boarding for flights QF400 and above and that there can be for flights QF1-399.

On the international services which may offer it, I don't see the need for it anyway. The lounges are often good at announcing the flight from the 1st and Qantas Club lounges when economy boarding is nearly complete, such that you can swarm down to the gate , get on the plane and be away.

"What would make sense is that when the flight is full, they have pre-boarding, and when it is say <75% full, they just have a general call."

Those that would be eligable for pre boarding would tend to be eligable for lounge access. Which is better, to wait in a pleasant lounge or to sit on a plane.

QF boarding is pretty efficient and unlike the US carriers , I have never seen a problem getting carry on luggage stored.

Dave