The Australian Federal Police has been refused access to thousands of documents it seized from former senator Stephen Conroy last year, after the politician's claim of parliamentary privilege was upheld.
The senate privileges committee has ruled that the AFP's seizure of the documents amounted to "improper interference" in the duties of a senator.
It said the documents were "sufficiently closely connected to [Conroy's] parliamentary business to warrant protection".
The committee also criticised NBN Co's actions during the raids; one of its staff members took photos of the seized documents and sent them to a colleague offsite to help the AFP identify those that were relevant.
It said these actions may have helped NBN Co identify the leaker. Two NBN Co staff were stood down over the affair.
"Information gleaned during this filtering process is problematic. The transmission of such information to a third party increases the risk that it may be used for purposes beyond those authorised by the warrant," the committee wrote.
"The committee remains concerned at the potential that unauthorised use of this information may have adversely affected an NBN Co employee."
NBN Co has denied it relied on any of the documents identified in the raids to hunt down the leaker.
However it admitted to the committee that it commenced an active investigation of one of the two employees that were later stood down after sighting emails between the two and the Labor staffer during one of the raids.
Conroy had also asked the committee to investigate whether the AFP's actions were in contempt of parliament.
It said while it was clear "improper interference" had occurred, the threshold for contempt "is a high one" that requires "cogent evidence of an improper act or motive".
"The committee is always reluctant ... to recommend that a contempt be found in the absence of the requisite intent, and does not do so here."
A lower house privileges had previously found the documents were subject to parliamentary privilege.
The AFP took the documents as part of an investigation into the leaking of sensitive internal NBN Co documents late last year.
Two raids were conducted: one in May on the office of former Labor senator Conroy and the house of a staffer employed by Labor's Jason Clare, and a second in August when it raided the Department of Parliamentary Services.
The AFP has been unable to access the documents since Conroy made the claim of parliamentary privilege.
The first leaks arose in November and December 2015, with separate files revealing NBN Co was considering overbuilding the degraded Optus HFC network - a move it later confirmed - and that it was facing a remediation bill for Telstra's copper of ten times more than expected.
A February leak of an internal progress report revealed the NBN was running behind on its fibre-to-the-node rollout, and a March leak indicated NBN Co's plans for fibre-to-the-distribution point, also later confirmed.
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