Why are the leaders of the Australian political parties so prone to being toppled?

All the answers given here have been excellent. I particularly liked Kai Neagle’s.

Several factors have contributed to Australia recently turning into postwar Italy, and most of them have already been pointed out.

• Labor has always been factionalised. The Liberals have become much more factionalised recently, with the resurgence of the reactionary right.
• Both parties have moved to The Mushy Centre. As a result, there is not a lot of sunlight between them, and there is pressure on them from their extremes: from the Greens, and from One Nation and other right wing populists.
• This has made the parties much more managerial than ideological, and accordingly much more prone to panic at poll results rather than sticking it out. If you don’t have an ideology, the only reason you are in power is to stay in power.
• Labor as a movement has suffered much more from the Twilight of the Ideologies, the demise of socialism, and the Hawke-Keating neoliberal reforms. So the cracks were always going to show there first.
• Labor was also structurally more prone to do this kind of thing, to begin with.
• Pundits at the time talked of federal Labor contracting Sussex Street disease—referring to NSW Labor, which has always been much more ruthless.
• The unionist Paul Howes, who was instrumental in toppling Rudd for Gillard, was derided as one of Labor’s faceless men. The insult is 50 years old: it comes from Menzies. The only difference with Howes is that he didn’t stay faceless: he gave TV time to anyone who would ask.

Many of these factors are shared throughout the Western world, and other answers have already mentioned them. They don’t explain why Australia has remained unstable. Others have brought up procedural reasons, which are beyond my expertise. I’ll offer a simpler reason.

Precedent.

Yes, the party leader is leader only by the grace of the party room. But toppling a sitting prime minister used to be Unthinkable. And the country was shell shocked when Rudd was toppled. I was in Melbourne’s Fed Square when it happened, and I remember dozens of us staring mouths agape at the TV screens.

Once it happened, the unthinkable became thinkable. And eventually, expected.