The revelation, contained in documents released under freedom of information laws, casts fresh doubt over claims by the Australian Federal Police that it did not investigate the unauthorised disclosure given "the large number of potential suspects".

The leak, which Australia's top spy decried as "seriously damaging" and Labor alleged was orchestrated by the Morrison government to discredit laws to fast track asylum seeker medical transfers, was referred to police by the Department of Home Affairs earlier this year.

The Sydney Morning Herald and The Age revealed in June that police evaluated the referral but opted against launching a full investigation "due to the limited prospects of identifying a suspect".

The decision stands in contrast to two other cases where confidential information was published, which culminated in dramatic raids on the home of a senior News Corp journalist in Canberra and the ABC's Sydney headquarters.

Cabinet-level advice about the potential impacts of a crossbench bill to bring sick asylum seekers on Manus Island and Nauru to the mainland was splashed across the front page of the Australian ahead of the legislation coming to a vote in February.

New documents show the secret material was prepared in December 2018 and sent to just 11 people or offices including Home Affairs Minister Peter Dutton, department secretary Mike Pezzullo, the department's legal counsel, three deputy secretaries and Border Force commissioner Michael Outram.

A number of other agencies were consulted in the preparation of the advice but nowhere in the documents is it stated that those agencies had access to the final version of the leaked material.

In referring the leak to police, Mr Pezzullo said the impact on his agency had been "critical" - the highest possible level.

Mr Pezzullo also said he was "agreeable" to the Department of Home Affairs conducting a joint investigation with the AFP to find the leaker and noted his officials had already started auditing the IT logs to determine who was responsible for the leak.

But an AFP case note shows police decided in less than a month that they would not investigate, citing the audit of the IT logs.

"The results of this [audit] indicated the list of addressees, and therefore suspects, is greater than 200," the case note reads.

About 171 were from Home Affairs while the rest were from the Australian Government Solicitor, Border Force and other sources redacted in the FOI.

"Given the large volume of potential suspects, the time and resources required to effectively investigate the matter would not be an efficient or effective use of AFP resources," the AFP case note concluded.

"Other than the identified volume of potential suspects, the AFP were unable to identify further avenues of inquiry to progress the investigation."

Labor will seize on the documents to support its belief that the AFP did not investigate because it could embarrass the Morrison government - a claim outgoing commissioner Andrew Colvin has denied.

Opposition home affairs spokeswoman Kristina Keneally said Mr Dutton had "gone silent" on the leak.

"Given that the AFP never commenced an investigation, potential suspects responsible for the leaks were never questioned, in spite of the fact it seems a small number of people had access to the final documents," she said.

"Mr Dutton also needs to come clean and explain what the internal investigation within his own department found about the leaks. Does he have confidence these classified documents weren't leaked by his department or his office?"

Mr Dutton has previously denied being behind the leak, which infuriated Australian Security Intelligence Organisation director-general Duncan Lewis.

The AFP case note clearly states the leak may have been a crime under the Crimes Act and the Criminal Code Act.

Seven AFP officers raided the home of News Corp reporter Annika Smethurst in June while six entered the ABC's Ultimo headquarters over separate stories published in 2017 and 2018.

Smethurst's report revealed top bureaucrats had discussed giving Australia's cyber spies broader powers. The ABC's stories used leaked Defence information to accuse Australia's elite special forces in Afghanistan of killing unarmed men and children.

The journalists and media organisations subject to the search warrants may have committed a crime by publishing the sensitive information and are the focus of police investigations.

The medical transfer laws passed Parliament against the wishes of the government in February. The Coalition is working to repeal the bill following its May election victory.