Currently you can book tickets on Qantas from both Melbourne and Sydney to Los Angeles for as low as $444 roundtrip. Available dates range from October, 2015 all the way through March, 2016.
This is a fantastic price, however note that you must originate in Australia to get this fare. If you are interested in booking, you can search and book on Priceline. I was able to find several available dates without too much work.
For more sample dates and info, see this Dan’s Deals post.