August 5 1914 At around 12:40 Melbourne time, the first shot fired by British empire troops were used to prevent the escape of the SS Pfalz.
No. 6 gun of the battery at Fort Nepean in the Mornington Peninsula, fired the first shot across the bows as the Pfalz was steaming out of Port Philip Bay.
http://www.thefirstshot.com.au/full-page/