The SAS corporal and Victoria Cross recipient, who has sued media outlets for defamation over reports of his alleged conduct while deployed in Afghanistan, applied to access diary entries of Major General Paul Brereton "evidencing dealings" with journalist Chris Masters for a book about Australia's involvement in the war.
Mr Roberts-Smith was blocked from accessing three diary entries and a calendar entry.
The Office of the Inspector-General of the Australian Defence Force (IGADF), under Maj Gen Brereton as assistant inspector-general, had undertaken the Afghanistan Inquiry to see whether there was any truth to "persistent rumours of criminal or unlawful conduct" by Australian soldiers.
Access to the entries was refused on grounds it could prejudice the investigation and would be contrary to Maj Gen Brereton's legal direction.
Mr Roberts-Smith unsuccessfully applied for an internal review.
A second application under the Freedom of Information Act to the Information Commissioner reiterated disclosure would be contrary to legal requirements in August 2022.
The decision then went to the Administrative Appeals Tribunal (AAT), however it has maintained Mr Roberts-Smith cannot access the documents.
The IGADF had abandoned a claim that providing them could prejudice an investigation, which the tribunal said was presumably because the Afghanistan Inquiry had ended.
However, it maintained giving Mr Roberts-Smith access to the sought material could reveal the identities of confidential informants and would be contrary to the public interest.
He argued there were restrictions on disclosing evidence given to an inquiry and the documents it received, but not on the disclosure of the identities of people who had met with the inspector-general's office.
The previous rejections were beyond the regulatory power available and not validly issued, he submitted.
The tribunal noted he ultimately accepted it did not have the power to determine the validity of prior directions.
It was upheld that disclosing the diary entries and the calendar entry would involve unreasonable disclosures of personal information and would be contrary to the public interest.
Mr Roberts-Smith argued there would be no prejudice to ongoing investigations authorities are conducting and the tribunal agreed.
However, if the information was disclosed it would likely discourage people assisting future inquiries.
"The public interest is not served by permitting a person's confidential participation in an inquiry of the kind underlying this dispute, or of similar or analogous kinds, to be disclosed," AAT deputy president Tom Thawley wrote in a decision published on Wednesday.
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