The cost of a controversial security fence being installed outside Canberra's Parliament House has to be kept secret because the price tag could aid a terrorist attack, according to the federal government.

Speaker Tony Smith and Senate President Stephen Parry have refused to release information about the 2.6 metre steel fence since last year, when plans for security upgrades and fortification across the sloping lawns were rushed through Parliament.

At Senate estimates hearings on Monday, officials from the Department of Parliamentary Services confirmed for the first time the location of the fence would be about one third of the distance up the famous lawn from street level.

When asked how much of the $60 million in security upgrades would be spent on the fence, Senator Parry said the information was sensitive and could only be explained in secret hearings.