On the centenary on Sunday of the sinking of the German warship SMS Emden by the light cruiser HMAS Sydney at the Battle of Cocos, Governor-General Sir Peter Cosgrove will praise Australia's achievement in what was its first naval victory.

But he also has great praise for the Emden's Captain Karl von Muller, who became known as "the last gentleman of war".

Sir Peter's speech to dignitaries and both Australian and German families on Cocos Island in the Indian Ocean describes the event as  "one of the first and most decisive battles of the Great War".

It was a testing initiation for a young navy and a young nation. When the seas had settled, 140 sailors were dead – 136 from the sunken Emden and four from HMAS Sydney.

"Indeed, much of the pride and passion we see in the Royal Australian Navy today can be traced to the endeavour and bravery shown by the crew of the Sydney," Sir Peter's speech reads.

"For its part, the Emden had already proven its credentials. Time and again, Captain Karl vonMuller and his crew had prevailed in foreign waters.

"For months the Emden had patrolled this region, with a ruthless tactical nous and an unparalleled humanitarian spirit.

"It had clinically sunk or captured 24 commercial vessels and destroyed Russian and French warships. Yet it did all this while only inflicting one death.

"Captain vonMuller achieved military success, but he did so with a great humanity and a genuine respect for the sanctity of human life. To be remembered alongside his crew as the 'Last Gentlemen of War' is something he, his family, and the German people can be very proud of.

Michael Colless and sister Stephanie McCarthy – whose father, Alan Colless from Northbridge, was a Boy First Class on the HMAS Sydney – will attend a service at Bradleys Head in Sydney on Sunday. Alan Colless joined the navy at 15 and volunteered to join the crew of Sydney shortly after Australia entered the war.

Historian Charles Bean names Mr Colless in his Official History of Australia in the War 1914-1918, along with another couple of boys, praising them for sticking to their duties despite the ferocity of the battle.

"We think he [my father] may have been a gunner's mate because he was working in a zone of great danger and his conduct was described as splendid," Michael Colless said.

"My father was a young boy, there were about 60 boys under the age of 18 in the crew. Young boys serving a young nation about to be tested in war on an international stage. They performed remarkably and pulled off the first naval victory of the war for Australia.

"My father and his old shipmates when they got together were always very respectful of the German sailors. That feeling of mutual respect is still there today, stronger than ever."

Rienhold Janssen and sister Elisabeth Hafkemeyer from Germany were travelling to the Cocos Island to remember their grandfather, Petty Officer 2nd class Bernhardt Huelsmann-Emden, who survived the attack.

Speaking at the Western Australian Maritime Museum in Perth, while attending an exhibition titled The Last Gentleman of War, Mr Janssen said his grandfather was working in a machine room that was nearly   destroyed. He climbed out on to the deck and was rescued.

Survivors were later allowed to add "Emden" to their names.

"It will be quite emotional. I never expected that," Mr Janssen said.  "He was a prisoner of war and his health suffered afterwards, because of that. We have come here as tourists and have been made welcome. The world has changed."