Qantas doesn't provide a seat map through its website. You are able to print off an aircraft layout in PDF format (when making the booking, click on the flight number and then the aircraft type you are travelling on).

If you want to view a seat map for a flight that you haven't booked, you can go to www.itn.net and complete a dummy booking. Towards the end it will show you what seats are and aren't available.

If you have booked the flight, go to www.checkmytrip.com. Enter your booking reference and surname and it will list your flight. Click on the "+" button at the bottom of the box for each flight and it will open out to include your seat allocation. Click on your seat allocation and the seat map should open up. I say should as sometimes it will and wont show you. Just keep trying.