When Power Goes Wrong · Evidence & Proof Ledger · Part 34

How do you prove that the unlawful act actually caused the loss claimed?

Liability is not enough. A claimant must connect the proved wrong to the particular loss and quantify that loss with evidence. Counterfactual reasoning, contemporaneous financial records and expert evidence often become decisive.

A public authority may have acted unlawfully or negligently and still not be responsible for every loss the claimant later suffered.

Causation asks what difference the wrongful act made. Loss asks what legally recoverable damage resulted and how it is quantified.

Chronology is not causation by itself.“The loss happened after the decision” is only the beginning. The claimant must show why the loss would probably not have occurred, or would have been materially different, absent the wrongful conduct, subject to the rules of the relevant cause of action.

Factual causationWhat would probably have happened without the wrongful act?

Legal scopeIs this type of loss within the responsibility imposed by the relevant duty or right?

QuantificationInvoices, valuations, accounts, bank records, lost-income evidence, market evidence and expert calculations.

Mitigation/alternativesWhat steps could reasonably have reduced the loss, and what independent events also contributed?

Daisley illustrates why damages require granular proof: the Supreme Court ultimately preserved a specific $90,000 property-value award while setting aside other damages on the limitation analysis.

Build damages as a ledger: each dollar category needs a legal basis, causal link, evidential source and calculation.

Current caseWhangārei District Council v Daisley [2026] NZSC 72.

Expert evidenceEvidence Act 2006, s 25.

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