Major Greenhalgh served as a rifle Company Commander on operations with 5th Battalion, The Royal Australian Regiment in Vietnam for eight months.
During operations Major Greenhalgh showed complete disregard for his own safety on numerous occasions when in contact with the enemy in order that his subordinates would receive the best possible support from him.
He set a high standard and fine example to his men at all times and thereby maintained the moral efficiency of his company at a high level.