Anywhere from $7 to $30 for most packages I've sent, but large packages can be quite a bit more. They use either weight or "cubic weight" (LxWxH in meters x250), whichever is higher, and distance to calculate the price. Prices are here
http://auspost.com.au/parcels-mail/parcel-post.html
You'll have to use the calculater to get exact numbers unless using prepaid satchels.
Satchels would be the easiest method for packaging and sending, but the bags aren't that big.