The Senate Privileges Committee has referred Carol Mills, the head of the Department of Parliamentary Services (DPS), to the Finance and Public Administration Legislation Committee for allegedly providing contradictory and misleading evidence, which the committee noted would be "a serious breach of accountability and probity".

The committee found it could not reconcile Ms Mills' evidence with a subsequent submission and documents provided by her department.

In February, DPS officials used images showing a DPS employee placing an envelope under the door of Labor Senator John Faulkner as part of an investigation into a staff management issue.

A furious Senator Faulkner later pursued the issue in Senate estimates hearings.

"This is the most serious breach," Senator Faulkner said.

"A senator in this Parliament has been spied on in that way as they go about the proper conduct of their duties."

The Senate Privileges Committee found there was improper interference, or attempted improper interference, with the free performance by Senator Faulkner in his duties.

In the May Senate hearings, Ms Mills said the spying matter had only come to her attention that day. But a submission by the department later said the footage showing the employee placing an envelope under Senator Faulkner's door had been communicated to Ms Mills in February.

Ms Mills, who is on leave, was previously regarded as the front-runner to become the top official in the UK House of Commons.

In August, Senate Clerk Rosemary Laing sent a remarkable email to British officials saying it would be "embarrassing" if Ms Mills was appointed to the post.

Ms Mills has been approached for comment.