I just came across what I think are great prices for flights from Perth to Beijing, in the best time of the year (weather wise) to visit the area - September. Sorry I can't check all the other possible dates, feel free to add your find to the comments.
Edit: actually, if you are careful to specify your return option those flights are shown on Adioso, that might be the quickest way to find dates that suit you.
What I did though: I checked other cities. If you want to explore China and travel overland a bit it is easy to fly back from several other destinations at very little extra cost ($22 extra to return from Shanghai or Chengdu). Just use the 'multi-leg' option to make the booking.
Have fun!
nice find. we traveled in September last year and the cost was over $1k because we didn't booked early.
some words of advice, September and October is the holiday season in china, so it is really.. really crowded.
Avoid visiting tourist attractions like Great Wall and Forbidden City during the Moon Cake festival on Sep 19th-21st and National Day - Oct 1st to Oct 7th this year unless off-course you're into crowded area stuff.