A 1,000-page manual on future security arrangements for Parliament House has been lost, but the Department of Parliamentary Services (DPS) argues the building's safety has not been compromised.

Labor senator Kimberley Kitching used Senate Estimates to grill DPS staff about the security breach, alleged to have happened because a contractor working on the building's security upgrades lost the document.

The document was lost in November and has not been found.

A new fence will stop patrons being able to walk onto the roof of Parliament House. ( ABC News: Matt Roberts )

Senate President Stephen Parry offered his Labor counterpart a private briefing on the situation to allay her concerns — but she wanted it discussed publicly.

"The fact that we are spending a lot of money, $126 million approximately, yet some of this may be undone because a security manual, a 1000-page security manual, was lost by another contractor I think is in the public interest," Senator Kitching told estimates.

DPS and Senator Parry argued it was an early draft of future security arrangements, allowing them to change the plans given the information had been lost.

The Senate president argued a full investigation by the Australian Federal Police and domestic intelligence agency ASIO was not necessary.

The Department of Parliamentary Services says the House is no more exposed to security breaches. ( ABC News: Nick Haggarty )

"You start off with the worst-case scenario — once the investigation was completed, our fears were allayed considerably," Senator Parry said.

"However, we still don't want to identify aspects of this so people don't go looking in areas we don't want people to go looking.

"We're comfortable at the moment that there's no compromise to the security arrangements at Parliament House."

Senator Kitching questioned DPS staff why they hired a private investigator in February to track down the document rather than police, and asked what consequences the contractor had faced.

"You're making an assumption about the seriousness of the matter," DPS official Paul Cooper replied.

He added the contractor has had to rethink its document handling processes.

Major security overhaul

The concession about the lost manual comes as Parliament House undergoes a major security overhaul, which has been criticised for affecting the design of the building.

Among the upgrades are fences restricting access to its iconic sloping lawns.

Installation of a new security fence began last month. ( AAP: Lukas Coch )

About 20 large trees have been ripped out, along with landscaping along the sides of the building to make way for fencing.

"This is probably the ugliest stage it will get, I think there'll be gradual improvement but it's going to remain ugly for a while," Senator Parry stated.

"I think you'll be pleasantly surprised."