Least expensive, QF, first class and Europe to SYD are words that are usually not associated. Why QF?
I assume this is a cash fare and not ff miles
For QF coded you are limited to QF ex LHR and EK ex various Eu locations.
You may find other carriers offer first class for less the QF.
The number of airlines having first class into Australia is small. A lot more with long haul international lie flat business class