You should be able to find out all the info on your granddad's service history through the Australian National Archives, link below.
The archives were completed for WWI about a year ago. They contain scanned images of all service documents relating to each individual that served. Other conflicts are also listed however they are not complete as yet.
If you have relatives that served in say WWII and their details aren't on the digital database you can contact the ANA and they can supply you with a copy of the documents they have.
The Archive site is free for the digital records, just do a search on the family name.
Australian National Archives website.
I'd like to take credit for being a web search guru but it was my wife who found all this info when she was searching for her Dad's WWI service history :)