The Pritzker prize winner said the erection of a 2.6-metre fence around Parliament's perimeter would betray the intentions of architect Romaldo Giurgola, whose design ensured the public could walk above their representatives.

"It's really a knee-jerk reaction. We're getting like a gated community: very American style. From an architectural point of view, I think it's terrible. It should not even be considered as an option," Mr Murcutt said.

"Romaldo Giurgola designed this building so that you had very good access to the people - so it expressed freedom, it didn't in any way express exclusivity. Putting a fence around it is putting a noose around it."

The proposal was endorsed by the Senate with the opposition's support on Thursday and work was expected to begin over the summer. In addition to the new fence, existing fences further up the hill will be made taller and 38 extra CCTV cameras will be installed.

Blueprints were being kept under wraps, with MPs briefed face-to-face by the Presiding Officers but denied any documents that could be leaked.

Mr Giurgola died in May and his moral rights to the building are now held in part by architect Pamille Berg. She echoed Speaker Tony Smith's assessment that "these security changes will obviously have a serious impact on the original design intent of Parliament House".

But Ms Berg conceded security needs had changed and there was a "difficult ongoing balance" between security and accessibility.

Security experts expressed scepticism about the changes, particularly the fences that would prevent people from walking up the grassy slopes to the top of the building.

Neil Fergus, CEO of the firm Intelligent Risks, which has advised governments and event organisers around the world, said federal Parliament security had tended to suffer from being piecemeal rather than holistic and integrated.

"They shouldn't be making knee-jerk decisions. They need to do a proper security risk assessment. Everything you do affects somewhere else. When you go out and do something that isn't properly thought through … you can actually create more security gaps than fixes," Mr Fergus said.

"I think it has been a problem since the new Parliament House opened. There have been fixes done on fixes incrementally over years."

Nick O'Brien, a former British counter-terrorism policeman now at Charles Sturt University, said fencing off the slopes around Parliament seemed to be "more about stopping civil protests than terrorism".

John Coyne, a former federal policeman now with the Australian Strategic Policy Institute, said fencing off the slopes was "a bit strange". While stressing the Australian Federal Police and ASIO may have identified specific intelligence that warranted fences, he said it was "very difficult to see a clear cause and effect".

"What would not allowing people on that grassy hill do? Putting someone on the side of a grassy hill is not going to give you any additional capability to attack someone."

Other architects who spoke to Fairfax Media on Thursday were universally scathing about the details that had been released.

Canberra architect Rodney Moss said Parliament House was falling victim to "security bracket creep". "To add layers and layers of security to the building will compromise the design intent that we all thought was so fantastic," he said.

"It becomes fortress Australia, which is the complete opposite to the way the building was envisaged." Mr Moss said other iconic buildings such as the Taj Mahal or the White House put perimeter security "a long way out from the buildings".