In what was seen as a test case for police procedure, Justice Christine Adamson ruled it unlawful for the commissioner, Andrew Scipione, or anyone on his behalf, to direct a Coffs Harbour officer, Constable Errol Baff, to answer questions about a critical incident.
Constable Baff had claimed the protection of the common law privilege not to incriminate himself.
Constable Baff discharged his weapon on May 31, 2011, shooting a Victorian woman.
The court heard he was immediately placed under suspicion of criminal responsibility for the shooting and refused to answer questions during the investigation which ensued.
Fairfax Media understands no charges have yet been laid.
During a hearing in the Supreme Court this week, counsel for the Commissioner of Police, Peter Bodor, QC, argued Constable Baff was required to answer questions for purposes of a departmental ''critical incident'' investigation, in accordance with sections of the Police Act 1990 and the Police Regulations 2008.
Mr Bodor submitted that Constable Baff's right of refusal was abrogated by virtue of the applicable clauses of the legislation, one of which is that upon becoming a police officer he undertook to obey all lawful orders.
''[Mr Bodor] argued that as the commissioner was lawfully entitled to ask questions of the plaintiff relating to the incident, the plaintiff was legally obliged to answer them, since he had undertaken to do so,'' Justice Adamsom summarised in her judgment.
But Justice Adamson rejected this analysis.
''Although he or his delegate are entitled to ask the plaintiff any question, the commissioner is not entitled to direct the plaintiff to answer a question once the privilege has been claimed,'' she ruled.
''On the basis of my construction of the act and regulations, the plaintiff has available to him the privilege against self-incrimination.
''Once it is claimed, any order directing him to answer would not be a lawful order …
''Since the plaintiff had undertaken only to obey lawful orders, he would not be in breach of the order or his undertaking if he refused to answer a question once he had claimed the privilege.''
Justice Adamson said Constable Baff remained exposed to criminal prosecution, notwithstanding the apparent conclusion of the criminal investigation - so his right should be preserved.
In making her orders, Justice Adamson said Constable Baff was ''entitled, in the exercise of his privilege against self-incrimination, to refuse to answer questions asked of him by the defendant'' and declared that ''any order directing the plaintiff to answer questions concerning the incident on 30/31 May 2011 is not a lawful order in circumstances where the plaintiff has claimed the privilege against self-incrimination.''
Costs were awarded in favour of Constable Baff.