When war broke out in August 1914, New Zealand had no formalised army or training ground. An army of volunteers was rapidly mobilised and various temporary training camps were set up around the country, particularly near Wellington as this was where the ships were to depart from. Trentham Camp, in the Hutt Valley, was built on the site of the Dominion Rifle Association, which already had shooting ranges. In the early days the camp was mostly made up of tents with a few permanent structures, as can be seen in this film footage. From March 1915, huts were built to replace them, but not fast enough to meet demand.

Trentham was the largest of the early training camps. Many of the soldiers who served at Gallipoli, particularly those of the Main Body, completed their training here. Although the largest, it was subject to immense overcrowding. The camp intended for fewer than 4,000 men held 7,000 by May 1915. Overcrowding led to poor health conditions. The medical facilities were inadequate and in July 1915 the camp was evacuated due to outbreaks of respiratory diseases, measles and spino-cerebral meningitis. Pressure was finally relieved in 1916 by the opening of the new Featherston Camp.