I'm looking for the Shapefile of the Australian Postcodes/area 4-digits. I would prefer an open source if possible.
According to the Australian Bureau of Statistics, exact geographic boundaries for the post codes are not well defined. Indeed, the data available from the ABS are re-released periodically. However for simple purposes you can probably find what you need from the ABS geography portal.
Always carefully read the data descriptions of course to make sure it's fit for your purpose.