Ernest Oswald Collett was born in 1897 at Malvern, Victoria, to Ernest Augustus Collett and Lucy Alice Collett (née Batten). He attended Tooronga Road State School in Malvern and joined the Senior Cadets. With the outbreak of the First World War, Ernest attempted to enlist but was initially rejected as not meeting the standards for chest size and eyesight. Undeterred, he later tried again, and after being declared fit by a doctor, succeeded in enlisting on 17 July 1915. The 18-year-old tramway clerk was alotted to the 8th Field Artillery Brigade (FAB), and assigned the rank of gunner and service number 19618. It was around this time that his older brother George also enlisted with the 8th FAB. Of the five boys born to Ernest and Lucy, they were the only two who had survived past infancy.

It would be a long wait for the brothers before leaving Australia, and George took the opportunity to marry his sweetheart, Alice. On 20 May 1916 the brothers embarked from Melbourne aboard HMAT Medic, arriving at Plymouth on 18 July. Ernest spent time in hospital with an illness, but by December he was ready to head to France, leaving Southampton on 30 December 1916.

Ernest and his brother served with the 29th Battery while at the front. In June 1917, George was promoted, leaving a position to be filled, and Ernest was promoted to the rank of bombardier in his stead. On 24 September 1917, Ernest was gassed in action near Zonnebeke. He was hospitalised at the 11th Stationary Hospital at Rouen, suffering the effects of mustard gas, and died on the evening of 28 September. He was 20 years old. Ernest was buried the following day at the nearby St. Sever Cemetery Extension, Haute-Normandie, France. His brother George was gassed in action the day of his burial. He was sent to the same hospital that treated Ernest, suffering from mustard gas poisoning and burns, from which he would later recover.