Ric Thorp, who is speaking in Canberra on Monday, said if parts of the Australian War Memorial could be demolished, it was not beyond the realms of possibility to think sections of Parliament House could eventually face the wrecking ball as well.

Anzac Hall is set to be knocked down as part of a $498 million redevelopment of the memorial.

"When you read about the government so quickly approving a half-a-billion-dollar expansion to the War Memorial, which involves knocking down a reasonably built, huge, highly awarded building by Denton Corker Marshall, I see little to prevent that happening [to Parliament House], to be very honest with you," Mr Thorp said.

"We live in difficult times with our political class, I suppose you might say."

Parliament House, which opened 31 years ago in May 1988, was designed by Mitchell/Giurgola & Thorp Architects. The design was chosen from 329 entries in an international competition to design Australia's new seat of government.

Mr Thorp said three decades after the building was opened, he was still happy with its design.

"I have maintained an enormous interest and some involvement in it ever since," he said.

"To be honest, the only thing that I ever have concerns about is the potential threat in the future, rather than anything that has happened to date.

"How future additions or alterations are going to be handled ... I just feel that there's no certainty going forward about that.

"We left behind, as part of our original competition submission, the required provision for future extension, but I would say that that's not necessarily being endorsed by the current occupants.

"As yet, nothing has happened, so it's only a concern for the future."

Mr Thorp is due to speak at an event organised by the Australian Institute of Architects' ACT chapter on Monday night at the ANU's University House, on the topic of untold stories surrounding the design and construction of Parliament House.

Journalist Helen Pitt, author of The House, which details what went on behind the scenes in building the Opera House, is also speaking. The event is being recorded for Radio National's Big Ideas program with Paul Barclay.

Now retired, Mr Thorp said Parliament still held the original designs from Mitchell/Giurgola & Thorp for any potential expansions. Apart from politicians, Parliament House is also a workplace for more than 5000 people during parliamentary sitting, and some say the time is ripe for an expansion.

"The competition entry conditions required that you indicate how you would expand the executive government wing, the House of Representatives wing and the Senate wing, and we had those things in place," Mr Thorp said.

Whether those extension plans will ever be used remains to be seen, but Mr Thorp does not believe politicians should decide on the best option if Parliament House is expanded.

"I would like to see something set up where there is a safeguard in place. It could be the various government architects of the states and territories being made responsible to approve any changes ... get someone who is out of the loop," he said.

As to the extra security that has been added to Parliament House, including a new fence, Mr Thorp said he was not overly concerned.

"I am a realist in terms of security. These things have to be done," he said.

"Anybody charged with the responsibility for making decisions at the Parliament House about security is going to take it seriously.

"I think there's been an improvement in the quality of the work being done. I think early on, in the life of the parliament, there were some pretty shoddy fences and things put up, but more recently they've been a much better design quality and suited much better to the building."

Romaldo Giurgola, who died in 2016, was the principal architect of Parliament House. In his will, he specified that the moral rights to the design of the building be managed by another of the Parliament House architects, Hal Guida, as well as public art consultant Pamille Berg - giving them the right to be informed of, and asked to comment on, any potential changes to the design.

Mr Guida said on Thursday he believed the Department of Parliamentary Services had acted in good faith to date when dealing with the moral rights administrators.

Mr Thorp agreed the department had the best intentions, but said he believed it could not withstand political force. He said he remembered the architects' original design for the members' and guests' dining room at Parliament House was changed without consultation at the last minute because some members did not agree with it.

"This can happen. People ask 'why doesn't anyone know this?' Well, I'll tell them," he said.

"Every opportunity I have, be it this talk or seminar, I will try to use to speak up for the future of the building. That it be treated with integrity and respect and overseen by outside forces or experts."

Big Ideas - Untold Stories: New Parliament House + Sydney Opera House, Monday, March 25 from 5pm to 7.3pm. The Common Room, University House, Australian National University.