There's a long running thread on this, although I'm not sure whether there is sufficiently up-to-date information on the current website setup:
http://www.flyertalk.com/forum/qanta...antas-com.html
An alternative is to get the total price from the QF website, preferably (if available) with the breakdown between base fare and TFC. Then go to ITA and price the flight using different booking classes until you find a match, which will then give you the fare basis code and therefore the booking class used. Or if you have full access to ExpertFlyer, look at the fare ladder there to see whether you can find the fare whose base fare matches the base fare being used by the QF website.