When Power Goes Wrong · Evidence & Proof Ledger · Part 32

When do you actually need an expert witness?

Expert opinion is admissible where it will substantially help the fact-finder on a matter within the expert's expertise. Experts owe an overriding duty to the court, not to the party paying them.

Experts should be used to prove genuinely specialised propositions—not to repeat the client's theory of the case.

Evidence Act s 25 permits expert opinion evidence where it is likely to substantially help the fact-finder understand other evidence or determine a consequential fact. Where an opinion relies on facts outside the expert's general body of knowledge, those facts must themselves be proved or judicially noticed.

High Court Rules r 9.43 and Schedule 4 require expert witnesses to comply with a code of conduct. The code says the expert has an overriding duty to assist the court impartially and is not an advocate for the party engaging them.

An expert cannot repair missing primary facts.An accountant may quantify loss, an engineer may explain causation, or a digital-forensics expert may authenticate systems data—but the expert's factual assumptions still need evidential support.

Use experts forSpecialised causation, valuation, accounting, medical, engineering, technical or forensic issues.

Do not use experts forGiving legal submissions, deciding credibility, or merely repeating facts ordinary witnesses can prove.

DutyIndependent assistance to the court.

Proof disciplineList every factual assumption in the expert opinion and identify the evidence proving it.

An expert opinion is only as strong as its expertise, methodology and factual foundation.

Evidence statuteEvidence Act 2006, s 25.

Expert codeHigh Court Rules 2016, r 9.43 and Schedule 4.

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