In Australian politics, a leadership spill (or simply spill) is a declaration that the leadership of a parliamentary party (also known as a caucus) is vacant and open for re-election. A spill may involve all leadership positions (leader and deputy leader in both houses), or just the leader. Where a rival to the existing leader calls for a spill, it may also be called a leadership challenge.
When a leadership vacancy arises due to the voluntary resignation or death of the incumbent, the resulting leadership ballot is not a leadership spill. Therefore, the 1968 Liberal Party leadership ballot after the disappearance of Harold Holt was not a leadership spill, despite the contest involving four candidates.
A leadership spill may result in a new hierarchy, or may confirm the status quo. If the party in question is in government, the election of a new leader will result in a new Prime Minister, Premier or Chief Minister; if the party is the opposition, the election of a new leader will result in a new Opposition Leader.
There were 72 leadership spills between 1970 and 2015; the phenomenon became increasingly common in the early 21st century. None occurred in the 1960s, 10 in the 1970s, 18 in the 1980s, 13 in the 1990s, and 31 between 2000 and 2015. Spills are three times more likely to occur when a party is in opposition compared to when it holds government. The frequent leadership spills and political instability in the 21st century – which saw five changes of Prime Ministers between 2010 and 2018 – has led to Australia being dubbed "coup capital of the democratic world".