Technically you aren't adding a segment or leg to the itinerary you are fundamentally changing the whole itinerary. Instead of it being BNE - LAX it's now MEL - LAX. Consequently the availability may very well vary between the two.
The availability should be the same but you need to be comparing apples to apples here. Are you saying that you can find MEL - LAX as a business classic award on the date of travel on the Qantas but can't using the rebooking tool? Simply looking at a specific sector (i.e. MEL > BNE) isn't a fair comparison.
One thing you can try asking is to see if you can change the itinerary to a multi-city:
- Sector 1: MEL > BNE on date of travel in business
- Sector 2: BNE > LAX on date of travel in business
Granted this will likely attract some additional points but you should be able to "see" all availability on MEL > BNE since that's now treated as a separate trip. However, since it's all on the same itinerary you should be able to check through bags and you'll also be protected in the event of a misconnection.
-RooFlyer88
