If you have no status, your odds are virtually nil, unless it's an unusually low load. The higher your status, the better your odds. SFO-SYD is a highly upgraded route.

There are many posts on FT about how to check upgrade inventory. In short, change your preferences on UA.com to "Expert," search for a saver award, and then look for "I" inventory greater than zero. Details are here: http://www.flyertalk.com/forum/showthread.php?t=113792

Good luck!