It actually IS, by "convention", P. Given there were two parties forming a Govt and the parties had different constituencies, the senior party allocated relevant ministeries to the Leader of the junior party who nominated the individuals from his party. I term this process "by convention" ie an agreement between the leaders re certain matters, etc.
Refer the ref I gave:
"When the non-Labor parties have been in power, the Prime Minister has advised the Governor-General on all Cabinet and ministerial appointments at his own discretion, although in practice he consults with senior colleagues in making appointments. When the Liberal Party and its predecessors (the Nationalist Party and the United Australia Party) have been in coalition with the National Party (or its predecessor the Country Party), the leader of the junior Coalition party has had the right to nominate his party's members of the Coalition ministry, and to be consulted by the Prime Minister on the allocation of their portfolios."
