In Westminster parliamentary systems, the government has a cabinet of ministers responsible for different policy areas, similar to the US presidential cabinet. The leader of the cabinet ministers is the prime minister, the head of government. The closest US analogy would probably be a cross between speaker of the house and house majority leader (the role of the speaker of the house in Westminster parliaments is somewhat different - in Australia the Speaker is a member of the House of Representatives elected by the House, and is responsible for the operation of the house, but does not get to decide what legislation gets put to a vote. That’s up to the governing party). The prime minister is supposed to be “first among equals”, but in reality the PM is the leader of the party and is hence the boss. The PM also de-facto exercises executive power, since the Governor-General who legally holds executive power only acts on the advice of the prime minister.