On 7 June 1917 at Messines, Belgium, during an attack on the enemy's position, and after his own company had suffered very heavy casualties, Captain Grieve located two hostile machine-guns which were holding up his advance. Under continuous heavy fire from the two guns, he succeeded in bombing and killing the two gun crews, then reorganized the remnants of his own company and gained his original objective. Captain Grieve set a splendid example and when he finally fell, wounded, the position had been secured.
Severely wounded in the shoulder by a sniper's bullet, Grieve was evacuated to England, and on recovery returned to his unit in October. However, due to subsequently suffering acute trench nephritis and double pneumonia, he was invalided to Australia in May 1918.