Enlisted in the NZEF and then requested a discharge for "domestic reasons" some weeks later. Embarked for the United Kingdom, and a commission in the Royal Field Artillery, on 27 April 1915.

"In London, on 16th January, Flight-Lieutenant Rollo Vavasour, R.F.C., died in hospital after an operation for appendicitis (writes the Post's correspondent). He arrived in England in June, 1915, and immediately obtained a commission in the Royal Field Artillery, transferring to the R.F.C. at the beginning of 1916, when he went with his squadron to the front and was immediately recognised to be a daring and resourceful pilot. In the autumn of 1916 he had to be invalided to England, and his health gave way. He had a number of thrilling encounters in the air. On one occasion, 10,000ft. up, he was attacked by a Fokker, and a bullet passed between his legs, which perforated the lubricating oil tank, but handling his machine with cool judgment he was able to land safely inside the British lines. In another encounter with a Fokker, Lieutenant Vavasour succeeded in bringing his opponent to the ground. After having completed his period of convalescence he was appointed flight instructor at an important training centre for pilots, but his health again became a source of trouble, and an operation for appendicitis was performed. Lieutenant Vavasour was the second son of Mr H. D. Vavasour, of Blenheim and his age was 23. He was educated at the Jesuit College, Sydney, being a member of one of the oldest English Roman Catholic families. The funeral took place at the Roman Catholic Cemetery, West Brompton, with military honors (sic).” [Marlborough Express Vol LI, Issue 68, 21 March 1917, p.4]