Answer
I believe during the Battle of Gallelopi(SP?)
EDIT
25 April 1915 would be the common answer, although there may be a case for the Boer War (in which Australian fought in the British services). With that caveat:
Trench warfare was first used by the Australians at Gallipoli during WW1. In fact, this was the first combat seen by Australian soldiers as an Australian Army, Australia having been declared a country rather than a UK Colony on 1 January 1901 ("Federation").
In a nutshell, Gallipoli was part of the Dardenelles campaign to capture what is now Instanbul. The main combatants were the Turks and some German advisors one side, and the Australians and New Zealanders (collectively the Australian and New Zealand Army Corps, or ANZACS) French, Indian, Newfoundland and Briitish troops on the other.
According to the Australian Department Of Veterans Affairs, there were over 392,000 causalties on both sides.
The date of the first landings was 25 April 1915. This date is commemorated each year as ANZAC Day, a national day of rememberance for all the fallen in Australia and New Zealand. For Australia, those fallen include those in the service of the British Army before Federation through to current casualties in Afghanistan.
END EDIT
They really were in a rock and a hard place. Truly amazing and worth the research if you are so inclined.
EDIT
The Australian War Memorial has details of this and other campaigns on line www.awm.gov.au
Ypres, Belgium
"Trench warfare was used in World War I"
chemical warfare was prevalent , later outlawed by the Geneva convention
Trench warfare is where people or armies battle in trenches
Trench warfare is famous because of its large scale use during World War 1. During the war, both sides used this type of warfare for protection.
World War 1 introduced aerial warfare with airplanes and trench warfare
Trench warfare was one of the new forms of warfare introduced during World War 1. Another new form of warfare was chemical warfare.
Ypres, Belgium
"Trench warfare was used in World War I"
chemical warfare was prevalent , later outlawed by the Geneva convention
Trench warfare. They would sit in trenches and shoot at the enemy trench
Trench warfare. They would sit in trenches and shoot at the enemy trench
Trench Warfare
Trench Warfare
trench warfare chemical warfare
trench warfare
war.