Section 43A(3) of the Civil Liability Act 2002 (NSW) requires an objective assessment of whether the authority's act or omission was so unreasonable that no authority could properly consider it reasonable — the decision-maker's subjective credibility, motives, and post-hoc conduct are irrelevant to this assessment. The test imposes a high threshold consistent with Wednesbury unreasonableness, and 'irrationality' should not be substituted as the test. On causation, where a warning sign was in place but did not change a driver's behaviour, the plaintiff must prove that a differently placed sign would have changed the driver's conduct — the existence of the actual sign that failed to alter behaviour is a critical factual consideration.
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