Agree with previous posters about Gumtree but it does depend on what you're looking for.
Bikeexchange.com.au is good for bicycles
Tradingpost.com.au is okay for buying goods
Domain.com.au or Realestate.com.au if you're looking for a place to rent or buy, or easyroommate.com.au or flatmatefinders.com.au for a housemate.
Ebay
Australia is my favorite place to get used furniture and appliances.
Unfortunately I don't think there's one comprehensive website here that's as widely and commonly used as Craigslist but Gumtree is the same idea and is getting more and more use.