For some time it has been assumed that public Interest Immunity (PII) applied to Australian Tribunals as well as Courts. In April 2016 the Full Court of the Federal Court of Australia (FCFCA) gave a reasoned decision as to the parameters within which that principle applies, in the case of Commissioner of Police, NSW v Qi Guang Guo  FCAFC 62
With its origins in the Common Law, PII is a rule permitting, and sometimes requiring, exclusion of relevant otherwise admissible evidence on the basis that doing so is injurious to the public interest, such as state security or certain criminal investigations.
Mr Guo had been the subject of a decision by the relevant Commonwealth Authority to decline him a (Permanent) Entry Permit Visa for reasons which included consideration of issues raised in respect of him by the NSW Police as to his character.
Mr Guo appeal to the Administrative Appeals Tribunals (AAT). In that appeal his counsel sought to question a relevant NSW Police Officer. Counsel for the NSW Police objected on the grounds of PII.
The AAT decided, in March 2016, that it was satisfied that common law PII did not apply in the relevant AAT proceedings and accordingly mandated certain action which included the relevant police officer giving evidence on certain conditions.
NSW Police appealed the decision to the FCFCA.
That the NSW Attorney General, had not made use of a provision allowing the NSW Attorney General to intervene in proceedings to seek exclusion for PII reasons did not, in the end, prove determinative.
FCFCA considered, in detail, relevant provisions of the AAT legislation, particularly section 36D Public Interest Questions
FCFCA considered earlier cases, as had the AAT.
In some early cases, it was expressed that PII was akin to a rule of evidence, not a rule of substantive law.
In the 1990s both the Commonwealth of Australia and the Australian states adopted new evidence legislation which dealt with, amongst other things, “Exclusion of Evidence of Matters of State” [section 130 Evidence Act).. Some saw and asserted that these characterised PII as an aspect of the rules of evidence, not a matter of substantive law
FCFCA considered PII to be a basic Common Law rule requiring clear statutory wording to be excluded.
The Court examined, in detail, legislation and legislative history of amendments to the Administrative Appeals tribunal legislation and the Evidence Act and concluded that common law PII was not excluded by relevant AAT provisions in respect of oral evidence in the circumstances envisaged in the case of Mr Guo.
FCFCA was of the view that Evidence Act provisions do not render PII merely a rule of evidence and held that it was a substantive rule of law only to be overridden by express statutory requirement not found to exist in the circumstances before the Court.