Simpson was moving through a jungle of large trees and dense bamboo undergrowth in rain and poor visibility in an area near the junction of the borders of Vietnam, Laos and Cambodia when one of his platoons became heavily engaged with the enemy. Simpson led the remainder of his company to its assistance. As the company moved forward, one of the platoon commanders, Australian Warrant Officer M.W.Gill, was seriously wounded and the assault began to falter. Simpson, in the face of heavy enemy fire, moved across open ground and carried Gill to safety. He returned to his company and then crawled forward to within ten metres of the enemy. From here he lobbed grenades into their positions. Simpson then ordered his company to withdraw and he and five indigenous soldiers covered the withdrawal.

In the first burst of fire from the next contact, Australian Warrant Officer A.M. Kelly, commanding 231 Company was wounded, and the battalion commander, Captain Green of the American Special Forces, was killed when he went to assist Kelly. Simpson quickly organised two platoons of soldiers and several advisers and led them to the location of the contact. Despite the fact that most of his soldiers had fled, Simpson moved forward through withering machine-gun fire in order to cover the initial evacuation of the casualties. The wounded, including Kelly, were evacuated but Simpson was unable to reach Green’s body because of heavy, accurate enemy fire. He then covered the evacuation of the wounded to the helicopter pad by placing himself between them and the enemy.