With the rank of Air Vice-Marshal Park took command of 11 Group, responsible for the fighter defence of London and southeast England, in April 1940. He organized fighter patrols over France during the Dunkirk evacuation and in the Battle of Britain his command took the brunt of the Luftwaffe's air attacks. He gained a reputation as a shrewd tactician and a fine leader of men. However, he became embroiled in an acrimonious dispute with Air Vice Marshal Trafford Leigh-Mallory, commander of 12 Group. His prickliness of character during the subsequent Big Wing controversy contributed to his removal from command at the end of the battle. He was sent to Training Command.
In July 1942 he returned to action, commanding the vital air defence of Malta. From there his squadrons participated in the North African and Sicilian campaigns. In 1945 he was appointed Allied Air Commander, South-East Asia, where he served until the end of the war. He retired in 1946 and returned to New Zealand where he lived until his death.