Change of party leader (Collier): Beginning of Willcock's period in office: 'Following the election in February 1936 caucus re-elected the ministry. Collier retired as party leader on 18 August 1936, and caucus elected Willcock as leader. On 19 August Collier resigned his commission, and Willcock was commissioned', Hughes and Graham, p. 233 (see 'Sources', below). Willcock became Premier of a majority Australian Labor Party government.
Change of party leader (Willcock): 'Willcock resigned as party leader on 31 July 1945 on grounds of ill-health. Caucus elected Wise leader and filled the vacancy in the ministry', Hughes and Graham, p. 234 (see 'Sources', below).
References: For a study of the style of parliamentary politics at the time, see David Black, 'Factionalism and Stability, 1911-1947', in David Black (editor), The House on the Hill: A History of the Parliament of Western Australia 1832-1990, pp 97-151, (Perth: Western Australian Parliamentary History Project, Parliament of Western Australia, 1991, ISBN 0730939839).
Note also, R F Pervan, 'Cabinet and Caucus: Labor in Western Australia, 1933-1947', University Studies in History, [University of Western Australia], 5(1) 1967: 1-37.
Summary information on Western Australian premiers from 1890 to 1982 and a short essay, 'The Premiers -- An Introductory Comment', can be found in Reid and Oliver (see 'Sources', below).
For a survey of Willcock's career, see Noelene Doohan, 'Willcock, John Collings (1879 - 1956)', Australian Dictionary of Biography, Volume 12, pp 494-495, (Melbourne: Melbourne University Press, 1990), on line at:
Colin A Hughes and B D Graham, A Handbook of Australian Government and Politics 1890-1964, (Canberra: Australian National University Press, 1968, SBN 708102700); John Mandy and David Black (editors),The Western Australian Parliamentary Handbook Centenary Edition, (Perth: Parliamentary History Project, Parliament of Western Australia, 1990, ISBN 0731697847); G S Reid and M R Oliver, The Premiers of Western Australia 1890-1982, (Perth: University of Western Australia Press, 1982, ISBN 0855642149).