You can't beat free.
Once you land in Sydney or Melbourne, join a free walking tour with a company that's earned TripAdvisor's Certificate of Excellence for 2013.
I'm Free Tours gets rave reviews for their guided walking tours given by friendly and knowledgeable locals. Simply meet at the designated starting point, as indicated on the website, and look for the guide in the bright green T-shirt that reads "Free Tour."
Tours run every day, rain or shine, with the exception of Christmas (Dec. 25). Guides accept tips for their services at the end of the tour, at each person's discretion.