I'm assuming the backup is on QF?

It shouldn't matter, they usually co-rate revenue passengers on BNE-LAX, MEL-LAX and SYD-LAX as the distance as almost the same - so it should be no different with a ZED. I have known people who have won MEL-LAX tickets and they have let them use them SYD-SFO.

To be sure call their staff travel office (they prefer if you call on a weekday rather than weekend):
Brisbane 07-3238-7920 (011-617-3238-7920 if calling from USA)
Sydney 02-8303-4444 (011-612-8303-4444 if calling from USA)

You can call the US number (800-227-4500) but they are contract staff and won't be very helpful.