I suggested this as 2 tickets because I'm not sure how Starwood would handle moving 50,000 Starwood points. As far as I know they only throw in 5,000 points for every 20,000 points you move, no proration. If that's the case, you'll get 120,000 Qantas points for 50,000 Starwood points, short 5,000 Qantas points.
It seems to me that for a single ticket you would have to exchange 55,000 AMEX points which convert to 27,500 Starwood points which then convert to 125,000 Qantas points.