Thanks! That was the problem - I had completely missed that my Google search had returned the US site for the baggage fees. Nice price difference! (Alas, I do need the extra bag this trip as I'm travelling for work with a large library of physical books. E-books don't work well for sharing with multiple people. )

Edit: I tried clicking the 'checked baggage' link on the reservation page - and it took me to the US website, with those different prices. Wrong, Qantas.

For $300 for one bag - might as well cough up the additional $99 and get Qantas Club membership!