Where multiple defendants each contribute to an indivisible injury through cumulative tortious conduct (such as successive asbestos exposure), each is liable even though no individual defendant's contribution was independently necessary or sufficient to cause the harm. The 'but for' test is not an essential prerequisite for establishing factual causation at common law. Expert pathological evidence on causation is admissible under s 79 of the Evidence Act even where the expert cannot identify the specific causal mechanism by which each individual period of exposure contributed to the disease, provided the opinion is rationally explained by reference to specialised knowledge.
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