Welcome to FT and the BA Board

You can only MFU itineraries which contain BA operated flights. You would therefore need to look to book LHR-(SIN)-SYD//LAX-LHR only (if that's allowed? I don't think it would be for an AmEx 2:1 redemption as they're different mileage zones, but for MFU you should be OK, I guess?), then sort out separate Oz-NZ-LAX flights on a different ticket. You should be able to redeem on Qantas (note, there's no such thing as Quantas!) as a oneworld Partner redemption with the EC (no MFU - straight redemption or paid tickets only) if you can find availability...this will also be key for your MFU anyway: good luck with the LHR-SYD availability you want, it's pretty tight normally. Note also Partner Awards are 'use or lose' once booked too.

Can I also recommend the ULTIMATE thread:

http://www.flyertalk.com/forum/showthread.php?t=582881

which will answer some MFU/general questions too.

Phil