The review, which was completed by engineering firm WSP, has resulted in changes to the design of the landmark London Circuit development, although none of the parties directly involved would disclose what those were.

Despite its significant stake in the project, the ACT government said it had only recently been made aware of "allegations" of a review, and has now launched its own investigation.

Terry Snow's Capital Property Group had scheduled a mid-2020 completion date for the development, which will include a 12 and a five-storey building.

The larger building will house private sector tenants, including law firm Minter Ellison and auditing giant KPMG, while the smaller block will accommodate 1700 ACT public servants.

The government awarded the tender for the project to the Snow family company in July 2016.

Construction work started in December 2017 and was progressing as planned until early this year.

At that point, WSP launched a review of the project's design as part of a wider assessment of developments involving engineering firm AWT.

WSP had previously acquired AWT, a Canberra-based company which had worked with Capital Property Group on a number of projects, including Constitution Place and the Brindabella Business Park.

The Canberra Times understands work stalled at Constitution Place for a six to eight-week period while the review was ongoing.

The delay has pushed backed the completion date for the 12-storey building, which will include a five star Adina hotel, until September-October. The five-storey building is due to be delivered on time.

WSP was the structural engineer behind Sydney's Opal Tower, which was evacuated on Christmas Eve after residents reported loud cracking sounds coming from the 36-storey building.

An independent report commissioned by the NSW Government into the complex found the building's structural support beams had been "under-designed", meaning they were vulnerable to "failure by shear compression and bursting".

In a statement following the report's release, WSP's Australia and New Zealand chief executive, Guy Templeton, said it did not "under-design" the structures.

"Put simply, it was not built as originally designed and changes made by others during construction have resulted in 'under-design' as the report states, or in other words, under-strength," Mr Templeton said in the statement.

The Canberra Times put a list of seven questions to WSP regarding the review, including whether it was prompted, at least in part, by the problems identified at Opal Tower.

It declined to comment.

However, Capital Property Group managing director Stephen Byron said his company had been assured that WSP's review was driven by its acquisition of AWT and "nothing else".

Mr Byron said the review had been completed and work was proceeding in accordance with an "updated design". He did not disclose what changes had been made.

He was supportive of WSP's review, saying it was reassuring that a company "taking responsibility for the structural design of the project would want to review those plans".

Asked if the company had confidence in the new design, structural integrity and overall built quality of the Constitution Place development, Mr Byron was unequivocal.

"We are proud to say we will own this development for many years to come and stand by its quality," he said.

"We are passionate about this project because it aims to add two, first-class, quality office buildings and workplaces, as well as a vibrant retail precinct with bars and restaurants to the Canberra Theatre district.

"It is poised to be an addition to the CBD that will help breathe new life into the city's heart."

An Access Canberra spokeswoman said the regulator had only recently been made aware of allegations that WSP had conducted a review.

The spokeswoman said the regulator had launched an investigation into the matter, but could not say how long it would take.

She said WSP had not been provided it with any information about a review of Constitution Place.