On 23 July 1944, aged 29, he was ordained as Priest of the Order of Missionaries of the Sacred Heart of Jesus by Archbishop Daniel Mannix.
Copas studied anthropology and tropical medicine at Sydney University in preparation for missionary work in Papua New Guinea, where he was based at Vunapope, from 1946 to 1951, and also served as a military chaplain from 1945 to 1948.
Alastair McIntosh, who was a Scottish VSO volunteer (1977–80) in St Peter's Extension School which he established at Kerema, records this (presently unpublished) memory of him: In his twilight years, Archbishop Copas continued to help the PNG people by bringing them to Queensland, Australia for life-changing surgeries.
These were people that had touched his heart and who could never hope to get the medical assistance in their home country.
These genuinely needy people would arrive on the Gold Coast and see truly amazing sights.