The Australian Federal Police (AFP) will have to return thousands of documents seized from a senator last year.

The officers raided the Melbourne office of Labor's Stephen Conroy and the home of a staffer while investigating leaks about NBN Co in May last year.

Mr Conroy had claimed privilege over the documents and asked the Senate to investigate whether the AFP acted in contempt of the Parliament.

The powerful Senate Privileges Committee has ruled that privilege does apply and they have to be returned.

It found the AFP's actions constituted "improper interference" in the duties of the senator.

It also raised concerns about a staff member of NBN Co taking photos of the documents during one of the raids.

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NBN Co told the committee it used other information to identify the staff involved in the leak, but the committee remained unconvinced.

"Information discovered during the execution of the Brunswick warrant may have assisted in identifying persons of interest in the investigation," the report stated.

"The committee remains concerned at the potential that unauthorised use of this information may have adversely affected an NBN Co employee."

While concerned about the potential misuse of the documents seized, the committee found the AFP's actions were not in contempt of Parliament.

"The threshold for a finding of contempt is a high one," the report stated.