The first shot of WWI was fired at a German merchant ship from a fort in Victoria, an event to be commemorated on its 100th anniversary on Tuesday.

The first shot fired by the British Empire in World War I wasn’t on the battlefields of Europe – it came from a windswept fort south of Melbourne, half a world away.

That moment – 100 years ago on Tuesday – will be marked with a ceremony at the former military base where a 24-year-old Australian soldier fired on the German merchant ship, SS Pfalz, which was attempting to flee Melbourne less than four hours after hostilities began.

The ceremony is one of a national series of events marking the centenary of World War I.

John Purdue, a sergeant with the army’s Royal Australian Garrison Artillery, was stationed at Fort Nepean at the tip of the Mornington Peninsula.

At 12.45pm on August 5, 1914, he was ordered to fire on the Pfalz to stop it from escaping Port Phillip Bay to the open sea.

The ship, which was carrying German consular officials and contraband, was captured and used as an Australian troop ship throughout the war.

Its crew was interned as prisoners of war.

A service to mark the 100th anniversary of John Purdue’s historic act will be held at the old officer cadet school at Portsea, now in the Point Nepean National Park, from 11am on Tuesday.

Sergeant Purdue’s grandchildren John Purdue, Carolyn Smyth and Louise Nicol, who will be among descendants and dignitaries at the ceremony, knew little of their grandfather’s role in history.

“Pa was a very strong individual. Very army. Very correct. Very stern,” John Purdue said. “[In those days] you didn’t sit down and have a conversation with your parents and your grandparents.”

Ms Smyth added: “Until we get older, too, the significance of an event like this doesn’t hit you, until a certain point in your life.”

Ms Nicol said the ceremony would strengthen their connection to their grandfather.

“When you’re growing up, it’s a little bit surreal to see a photograph of a man in a hat and medals on a wall,” she said.

“This will be part of our history, and our children’s history, forever.”

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