The ABC breached four editorial standards in stories by sacked reporter Mahmood Fazal, its ombudsman found.
The episodes involved have already been withdrawn by ABC News.
The ABC has said it accepts the report's seven recommendations and has started to implement them.
The ABC's Four Corners program had insufficient verification and fact-checking and a culture of "limiting knowledge" about anonymous sources, its independent ombudsman has found, identifying breaches of editorial standards in two stories by sacked reporter Mahmood Fazal.
The ombudsman's report, released this afternoon, found two episodes failed to meet editorial standards for accuracy, fair and honest dealing, and independence.
"Audiences were given a misleading impression of who key contributors were, the basis on which the ABC was relying on them and the level of confidence that could reasonably be placed in their claims," the report said.
The ombudsman made seven recommendations, which include amending the ABC's editorial standard on anonymity, further legal and editorial training for the Four Corners team and more support for less experienced reporters.
The ABC says it accepts the recommendations in full and has begun implementing them.
ABC News had already removed the two episodes covered by the ombudsman's review — Cocaine Nation and Meth Highway — after its own investigation into the anonymous interviewees.
In total, three episodes of Four Corners and two of Background Briefing by Fazal have already been withdrawn by the ABC, with a short segment edited out of another Four Corners episode.
Last month, the ABC said its own review found verification policies on anonymous interviewees were "not followed to a level that meets our editorial standards".
"The ABC is committed to journalism of the highest standard. Maintaining the trust of Australians depends on upholding those standards and we take any concerns about our journalism seriously," it said.
Fazal, a former member of the Mongols bikie club, had his employment terminated in May after an ABC investigation into his appearance on a podcast with a former underworld figure last year.
He had launched an unfair dismissal claim in the Fair Work Commission, but his hearing was removed from the schedule. The ABC said it was aware Fazal was no longer pursuing his claim.
In a social media video post last month, Fazal said he felt like the ABC was "trying everything in their power to stop me from ever working again".
"Even though all my work was approved across different teams and departments, cross-checked and met the standards before broadcast, they've decided there's now different standards that are retrospectively being applied to all of my stories," he said.
"They want to make me a scapegoat? Don't give a s***. I won't stop."
He has previously said he was hired by the ABC for his access to certain sources, but that proximity had then been used against him.
Previously, Fazal's reporting appeared in VICE, The Guardian, Rolling Stone, The Sydney Morning Herald, The Monthly and The Saturday Paper.
Last month, the ABC announced its head of investigations and current affairs, Jo Puccini, was leaving the public broadcaster after a 25-year career.
She had oversight of Four Corners and Background Briefing.
Other senior managers at the ABC at the time of Fazal's reporting, including news director Justin Stevens, have already left the broadcaster. The ABC's incoming news director Simon Robinson started in the role on September 14.
Last week, ABC managing director Hugh Marks rejected an Australian Communications and Media Authority finding that the public broadcaster breached impartiality obligations in an unrelated Four Corners program about the redevelopment of the Australian War Memorial.
ACMA said on Thursday it had found the program's March 2025 episode, Sacrifice, had "unduly favoured one perspective".
The ABC says it "stands behind the team" that created the episode.
"If this interpretation of the code is applied, including more widely across the industry, it could have a chilling effect on the body of investigative journalism that delivers the important stories that serve the public interest," Marks said in a statement.