Authority chief executive Malcolm Snow told a parliamentary committee on Thursday the 2.6 metre metal fence, about to be installed across the lawns of the building as part of a wider security upgrade, was not ideal but required because of security risks for MPs and visitors in high profile setting.

The final design and exact location for the fence have been kept secret from the public, even as temporary construction fencing is in place on the sloping lawns.

Senate President Stephen Parry has previously said discussing details of the plan would add to security threats.

"The NCA's view is that we accept, based upon the advice from Parliament and the President and Speaker, that this is a matter which is of serious concern," Mr Snow said in response to questions from Canberra MP Gai Brodtmann.

"We acknowledge... that it's not ideal that we introduce this element but I think this is where being pragmatic about it, if Parliament feels there are these clear security risks that we need to do something about it."

"Our concern... is to ensure any new element is designed appropriately. We did strongly support the idea... that if that security threat or risk was to change or reduce, that the thing can be easily removed in the future."

Heritage architect Eric Martin told The Canberra Times Speaker Tony Smith and Senator Parry needed to show the public how the Capital Hill vista would be impacted by construction and the final design.

"I find the whole process unusual and strange," he said.

"Whatever is being built will be highly visible so it seems strange they can't give some impression of what it's going to look like through a design process.

"You can hide the structural details and the security side of it but you won't be able to hide the location and basic design because it will be around for everyone to see. Not releasing any information is really unfortunate and lacks basic consultation."

Visiting Parliament House on Thursday from the United States, Geoffrey Robinson and Barbara Aurand said the temporary fencing was the first thing they talked about.

"It's almost anti-democratic that you would have those fences put in but times have changed and unfortunately things like that have been installed in places around the world now. It is unfortunate because it almost gives a symbol that the terrorists have won by doing that," Mr Robinson said.

Ms Aurand described the construction equipment as unattractive.

"I think visually it interrupts the flow and the majesty of it. It's a symbolism thing of the tyranny of the world and how crazy it's gotten and how democracies are struggling to survive and to control violence without gun control," she said.