On 19 March, 1932, New South Wales Premier Jack Lang opened the Sydney Harbour Bridge. Unfortunately for Lang, at the moment of cutting the ribbon, a right-wing, paramilitary member called Francis de Groot upstaged him by riding up on horseback and using a ceremonial sword to cut the ribbon.
de Groot's action was in protest against Lang and his Socialist government, on behalf of the New Guard, the quasi-Fascist group that at the time threatened to act against Lang. The New Guard had the capacity and intention of launching a massive uprising against Lang unless he resigned.
Fortunately for Australia, shortly after the incident, Lang was dismissed by Governor Sir Philip Game, after Lang refused to allow the federal government access to state funds. The crisis over, and Lang out of office, the New Guard disbanded.
This ATL proposes a different series events. In this timeline, de Groot's actions were more dramatic. Rather than simply cutting the ribbon, de Groot chose to strike a "blow for freedom" and used his sword to strike Lang and assassinate him. The attack saw the beginning of a New Guard uprising and the beginnings of a Fascist coup in New South Wales. So far as anybody knows, the New Guard never planned to kill Lang. They did, however, plan to kidnap him and use that action as a catalyst for a coup. Therefore, the scenario is not totally implausible. We will create a POD for the timeline and place it sometime in early 1932, when the New Guard decide, rather than kidnap Lang, to kill him instead. de Groot is chosen for the task, but his membership of the New Guard is revoked, so that the New Guard can deny culpability. When de Groot's fatal blow strikes Premier Lang, it is the catalyst for a fascist-coup in Sydney and mass violence. The federal government deploys troops to control the situation, but many commanders are New Guard members and rebel, instead opting to support the coup. This leads to a standoff between New South Wales and the rest of the Commonwealth, ultimately leading to the largest state's secession and the (temporary) collapse of the Commonwealth and return to direct British rule - at least until World War II intervenes.