We used an agent, but more because we had about 4 different visa we could apply for and wanted the quickest/easiest one that would give us a good chance of getting in (dh family here & he really wanted to be here too).
Have a look at \link{http://www.immi.gov.au/this} and it will tel you all you nee to know. tbh, we found it much less hassel to use an agent. We applied for former resident and we have tocollect info to prove dh used to live here before, that his family was here, that they kept in contact, had visited each other in the past, that he had flown back for holidays etc. Our agent gave us a list of what dimia expected from us. Then he managed to get all the evidence back as they will sometimes keep it. It's a very individual thing, well worth the money we thought.