They're dumping domestic overproduction on foreign markets. You do that long enough and you can weaken your foreign competitors in their domestic strongholds. This can destroy high-value domestic jobs, lead to atrophy of expertise, and is especially bad to "strategic" sectors of the domestic industrial base.
Tariffs are a tool to protect domestic markets against dumping. Every other nation's automotive companies are free to compete in Western markets because they're not actively engaging in dumping. Their competition strengthens domestic rivals because it is fair and on equal footing.
Australia doesn't care because it doesn't have a domestic automotive industry to protect, so Chinese overproduction is essentially just foreign subsidized stimulus to the Australian population.