Hanson-Young spying 'foolish': dept

Top immigration officials would have taken no longer than a nanosecond to discard the "foolish" action of spying on Sarah Hanson-Young, the department boss says.

The immigration department confirmed at least one security guard working on Nauru acted without authority and unofficially when spying on the Greens senator during her visit in 2013.

The Wilson Security shift supervisor was disciplined after the contractor, which runs security at the Australian offshore asylum-seeker detention centre on the island, discovered the spying at Senator Hanson-Young's hotel.

Department secretary Michael Pezzullo, who heard about the allegations in the media, said clearly someone had taken it upon themselves to track Senator Hanson-Young and had been dealt with.

But the matter should have been escalated to senior management at the time.

Despite not being at the helm in 2013, Mr Pezzullo is confident management would have quashed any spying plan immediately.

"It would have taken no more than a nanosecond," he told a parliamentary inquiry into allegations of abuse at the centre on Nauru on Tuesday.

"I think they would have very quickly jumped at any such suggestion had senior members of the department been consulted about such a foolish action."

Following revelations Senator Hanson-Young had been spied on allegedly using the codename The Raven, Prime Minister Tony Abbott excused the monitoring as an attempt to keep her safe.

However, Mr Pezzullo said if there was any serious concern about the senator's safety, she should have been told and the department should have been consulted.

"Because it seems very, very far fetched," Mr Pezzullo said.

Mr Pezzullo said he had no idea why an employee took it upon themselves to spy on the senator, but would ask contractors on the island.

"I don't wish to impugn the person at all, he might have had a brain explosion, he might have thought he was operating under diligent action," he said.

The department denied the incident proved it had little control over its contractors.