The 2 biggest I know of are:

Yow - http://www.yowconference.com.au/

Microsoft Tech Ed - http://australia.msteched.com/

Even though I am from Melbourne and have bias there, I would have to say that Sydney is the right place to go to a conference if you can only do one. If you can do two, Melbourne would be your next best.

If the cost of travel to Australia is big, then make the most of it while here, and do at least Sydney and Melbourne!