Originally Posted by number_6
This is the downside of buying a codeshare. When you book BA codeshare on QF metal, you become a customer of BA and not of QF. The ticket revenue goest to BA, who then buys the seat from QF (presumably for less) and pockets the difference. Any upgrades etc. are possible only using BAEC. Almost all FF plans prohibit upgrades on codeshares (for the obvious reason, it would cost real money for BA to buy a J seat from QF). You lose lots of little things when you buy a codeshare, for example seat selection is worse on the codeshare (the operating airline keeps the best seats for its own use) and op-ups almost never happen on the codeshare side. Sometimes codeshares are a lot cheaper (this varies by route and is a marketing decision by the airline). Most US-based airline codeshares are more expensive (e.g. AA codeshare on LAX-SYD); this is because their primary market are US govt agencies and contractors who must fly US airline whenever offered on a route, thus they cannot fly QF or NZ as long as UA is operating LAX-SYD, hence the AA codeshare on QF which they can fly.
Ironically as SIN-BNE will be classed as a JSA route so whatever profit is split between the two carriers regardless of who the bookings go through. Remember this is also the reason that BA was cross with QF when BA was offering and BA was filling up its J class compared to QF's then inferior product yet BA had to share the profit with QF!