William de Mars

Admitted as a legal practitioner in 1993 (NSW, WA and NT) 
Admitted as a barrister in 2018

Professional Background

William came to the bar after a long career as a solicitor and solicitor advocate for Legal Aid NSW and Aboriginal Legal Services in Western Australia and the Northern Territory.  As well as managing the Coronial Inquest Unit at Legal Aid NSW, he practised extensively as a criminal law advocate and representing claimant groups in  Native Title matters.  He has also worked as a policy advisor to the Federal Human Rights Commissioner.

He is on the Legal Aid specialist Appellate Criminal Law panel and appears in the Court of Criminal Appeal both led and unled, and all other types of criminal proceedings.

He also appears regularly in Coronial Inquests and Commissions of Inquiry both as Counsel Assisting and for interested parties (both families and institutions).

He also appears and advises in Native Title matters including both determination and compensation proceedings, and has had extensive experience working with applicant groups interstate including in Western Australia and South Australia.

He has also previously served as a part time member of the Mental Health Review Tribunal (NSW).


Principal areas of Practice

• Commissions of Inquiry and Coronial Inquests
• Native Title
• Criminal Law

Commissions of Inquiry and Inquests

• Counsel Assisting the Special Commission of Inquiry into LGBTIQ Hate Crime (2022-2023)
• Counsel Assisting the Special Commission of Inquiry into ‘Ice’ (custodial issues) (2019)
• Counsel Assisting the Coroner (various, 2019-present) including death in custody, medical and police operation matters
• Counsel for families and institutional parties in coronial inquests (various)

Native Title

• Far South West Coast Sea Claim (SA) (Federal Court 2022) (for Applicant)
• Daniel v State of Western Australia [2003] FCA 666 (Native Title determination)
• Daniel v State of Western Australia (2000) 178 ALR 542 (expert evidence in Native Title)

Criminal Law
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• R v (name restricted) (2020-2021, junior to R Wilson SC in two complex murder trials)
• Prasad v R [2020] NSWCCA 349 (with C Smith SC, conviction appeal re s65(2)(c) Evidence Act)
• Attorney General for NSW v XX [2018] NSWCCA 198 (instructor, for respondent to first application in Australia under “double jeopardy” legislation)

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For further details or enquiries regarding William de Mars' areas of expertise, fees and terms of engagement please contact his Clerk.

CONTACT HERE