I'm not familiar with NW partners, but Qantas is an AA partner. Your best bet would be to book a ticket with an AA flight number on Qantas to get 100% mileage credit. If you book it as a Qantas flight number, you'll only get 70% mileage credit.
You might post separately on the AA and NW boards and see what feedback you get there (many more people in a position to offer comments there).