Hi Melanie and welcome to BE.
Yes you are right about the compulsory school age, actually it's 6 years and 6 months.
Usually though, most children go to pre school before starting primary. Increasingly as mothers have to work in order to contribute to a mortgage, babies are put into daycare, and then into kindy and then onto pre school, so they are well used to the system before primary school age.
This link to the education dept for WA might help explain it. http://www.aussiemove.com/city/city.asp?f=2&c=7